Could not locate call adapter for io.reactivex.Observable

解决：

 Retrofit.Builder builder = new Retrofit.Builder();

 builder.addCallAdapterFactory(RxJava2CallAdapterFactory.create());

  compile 'com.squareup.retrofit2:retrofit:2.3.0'

    compile 'com.squareup.retrofit2:converter-gson:2.3.0'

    compile 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'

    //rxandroid

    compile 'io.reactivex.rxjava2:rxandroid:2.0.1'

    compile 'io.reactivex.rxjava2:rxjava:2.0.1'