Retrofitting most likely has a transitive dependency on okhttpand uses 2.4.0.
You may try
compile ('com.squareup.retrofit:retrofit:2.0.0-beta1'){
exclude module: 'okhttp'
}
compile 'com.squareup.okhttp:okhttp:2.3.0'}
, okhttp, .
, Retrofit / OkHttp , .