title: Using RxJava 2 and Retrofit 2, adapter version issue
body: <p>I've added a new library module to existing application module in Android Studio. The main difference was adding RxJava 2 and Retrofit 2.
  After updating build.gradle of new module I started to get next error:</p>
  
  <blockquote>
    <blockquote>
      <p>Error:Execution failed for task ':app:transformResourcesWithMergeJavaResForBetaNewApiDebug'.
          com.android.build.api.transform.TransformException: com.android.builder.packaging.DuplicateFileException: Duplicate files
      copied in APK META-INF/rxjava.properties  File1:
      C:\Users\Gaket.gradle\caches\modules-2\files-2.1\io.reactivex.rxjava2\rxjava\2.0.2\cfccdd18cdfbe7b4773d42c9f3512eeafbe5cbf9\rxjava-2.0.2.jar
        File2:
      C:\Users\Gaket.gradle\caches\modules-2\files-2.1\io.reactivex\rxjava\1.1.5\ece7b5d0870e66d8226dab6dcf47a2b12afff061\rxjava-1.1.5.jar</p>
    </blockquote>
  </blockquote>
  
  <p>I see that there is some problem with RxJava (I want to try RxJava 2 and here I see both RxJava 1 and RxJava 2). The problem arises right after adding all dependencies, we do not use <code>RxJava</code> in our main application.</p>
  
  <p><strong>Update:</strong>
  As I mentioned below, I checked three topics (you can see it below). This issue: <a href="https://stackoverflow.com/questions/33923461/how-do-i-resolve-duplicate-files-copied-in-apk-meta-inf">possible duplicate</a> is similar to the one that I already mention below:  <a href="https://stackoverflow.com/questions/38811753/duplicate-files-copied-in-apk-meta-inf-on-many-libraries">this one</a>. Both of them have workarounds that do not take into account the real problem that I underlined in my answer. And the <em>answer is</em> "There is no production-ready adapter from Retrofit 2 to RxJava 2 at the moment" (full details described in answer below).</p>
  
  <p>I checked several topics: <a href="https://stackoverflow.com/questions/34282190/android-studio-1-5-1-duplicate-files-copied-in-apk-meta-inf-notice-txt-commons-c">first</a>  is some concrete solution for a concrete problem, <a href="https://stackoverflow.com/questions/40371315/migrate-from-rxandroid-1-x-to-2-x-rxjava-included">second</a> talks about <code>rxbindings</code>, but I don't use them, some others also have not resolved my problem or look like total workarounds <a href="https://stackoverflow.com/questions/38811753/duplicate-files-copied-in-apk-meta-inf-on-many-libraries">this one</a>. I tried to google "rxjava 2 and retrofit 2" but haven't found a solution.</p>
  
  <p>Here is the <code>build.gradle</code> of library:</p>
  
  <pre><code>apply plugin: 'com.android.library'
  apply plugin: 'com.jakewharton.butterknife'
  
  android {
  compileSdkVersion 25
  buildToolsVersion "25.0.0"
  
  defaultConfig {
      minSdkVersion 15
      targetSdkVersion 24
      versionCode 1
      versionName "1.0"
  
      testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
  
  }
  buildTypes {
      release {
          minifyEnabled false
          proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
      }
  }
  }
  
  
  
  dependencies {
  compile fileTree(dir: 'libs', include: ['*.jar'])
  androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2',      {
      exclude group: 'com.android.support', module: 'support-annotations'
  })
  
  compile "com.android.support:appcompat-v7:$supportLibraryVersion"
  compile "com.android.support:recyclerview-v7:$supportLibraryVersion"
  
  compile 'io.reactivex.rxjava2:rxjava:2.0.2'
  compile 'io.reactivex.rxjava2:rxandroid:2.0.1'
  compile 'com.squareup.okhttp3:okhttp:3.5.0'
  compile 'com.squareup.retrofit2:retrofit:2.1.0'
  compile 'com.squareup.retrofit2:converter-gson:2.1.0'
  compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'
  compile "com.makeramen:roundedimageview:1.3.0"
  
  annotationProcessor 'com.jakewharton:butterknife-compiler:8.4.0'
  provided 'javax.annotation:jsr250-api:1.0'
  compile 'com.jakewharton:butterknife:8.4.0'
  
  testCompile 'junit:junit:4.12'
  
  }
  </code></pre>
  
  <p>The question is:<br>
    Why does this problem happen and what is the proper way to handle it?</p>