retrofit 2.0 xml simplexml converter issue while h

Here is my gradle file

apply plugin: 'com.android.application'
apply plugin: 'me.tatarka.retrolambda'
android {
    compileSdkVersion 23
    buildToolsVersion "23.0.2"

    defaultConfig {
        applicationId "com.demo.sample"
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:23.2.0'
    compile 'com.android.support:design:23.2.0'

    //Retrofit
    compile 'com.squareup.retrofit2:retrofit:2.0.0-beta4'
    //Retrofit support libraries
    compile 'com.squareup.retrofit2:converter-gson:2.0.0-beta4'
    //compile 'com.squareup.retrofit2:converter-simplexml:2.0.0-beta4'
    compile 'com.squareup.retrofit2:adapter-rxjava:2.0.0-beta4'
    compile 'com.squareup.okhttp3:logging-interceptor:3.0.1'
    //RxAndroid
    compile 'io.reactivex:rxandroid:1.1.0'
    //Explicit support for latest RxJava
    compile 'io.reactivex:rxjava:1.1.0'
    //Square dagger
    provided 'com.squareup.dagger:dagger-compiler:1.2.2'
    compile 'com.squareup.dagger:dagger:1.2.2'
    //Butterknife
    compile 'com.jakewharton:butterknife:6.1.0'
    //Picasso
    compile 'com.squareup.picasso:picasso:2.3.3'
    //Testing dependency
    testCompile 'junit:junit:4.12'
    testCompile 'org.assertj:assertj-core:2.1.0'
    testCompile 'org.mockito:mockito-core:2.0.23-beta'
}

If i un-comment line compile 'com.squareup.retrofit2:converter-simplexml:2.0.0-beta4' android studio throws nasty error at compile time

:app:transformClassesWithDexForDebug
trouble processing "javax/xml/XMLConstants.class":
Ill-advised or mistaken usage of a core class (java.* or javax.*)
when not building a core library.
This is often due to inadvertently including a core library file
in your application's project, when using an IDE (such as
Eclipse). If you are sure you're not intentionally defining a
core class, then this is the most likely explanation of what's
going on.
However, you might actually be trying to define a class in a core
namespace, the source of which you may have taken, for example,
from a non-Android virtual machine project. This will most
assuredly not work. At a minimum, it jeopardizes the
compatibility of your app with future versions of the platform.
It is also often of questionable legality.
If you really intend to build a core library -- which is only
appropriate as part of creating a full virtual machine
distribution, as opposed to compiling an application -- then use
the "--core-library" option to suppress this error message.
If you go ahead and use "--core-library" but are in fact
building an application, then be forewarned that your application
will still fail to build or run, at some point. Please be
prepared for angry customers who find, for example, that your
application ceases to function once they upgrade their operating
system. You will be to blame for this problem.
If you are legitimately using some code that happens to be in a
core package, then the easiest safe alternative you have is to
repackage that code. That is, move the classes in question into
your own package namespace. This means that they will never be in
conflict with core system classes. JarJar is a tool that may help
you in this endeavor. If you find that you cannot do this, then
that is an indication that the path you are on will ultimately
lead to pain, suffering, grief, and lamentation.
1 error; aborting
Error:Execution failed for task ':app:transformClassesWithDexForDebug'.
> com.android.ide.common.process.ProcessException: org.gradle.process.internal.ExecException: Process 'command '/Library/Java/JavaVirtualMachines/jdk1.8.0_74.jdk/Contents/Home/bin/java'' finished with non-zero exit value 1

I'm not understanding what the error actually means. Also, is there a way to get around this problem.

Seem like Simple XML has transitive dependencies which are already included in Android. See https://github.com/square/retrofit/issues/1536. We need to exclude three dependencies: stax:stax-api, stax:stax, and xpp3:xpp3. I managed to achieve this by using this compile statement in gradle

compile ('com.squareup.retrofit2:converter-simplexml:2.0.0-beta4'){
    exclude module: 'stax-api'
    exclude module: 'stax'
    exclude module: 'xpp3'
}