add dependencies for rxjava

app-build.gradle

apply plugin: 'me.tatarka.retrolambda'

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

dependnecies {
  compile 'io.reactivex.rxjava2:rxjava:2.0.+'
  compile 'io.reactivex.rxjava2:rxandroid:2.0.+'
  compile 'com.squareup.retrofit2:retrofit:2.1.0'
}

build.gradle

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
    
        // add this
        classpath 'me.tatarka:gradle-retrolambda:3.4.0'
    }
}