build.gradle

buildscript {
    ext.kotlin_version = '1.2.10'
    ext.rxjava2_version = '2.1.9'
    ext.retrofit_version = '2.3.0'
    ...
}
...
dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"

    // RxJava2
    compile "io.reactivex.rxjava2:rxjava:$rxjava2_version"

    // Retrofit
    compile "com.squareup.retrofit2:retrofit:$retrofit_version"
    compile "com.squareup.retrofit2:adapter-rxjava2:$retrofit_version"
    compile "com.squareup.retrofit2:converter-gson:$retrofit_version"

}
...