dep-6.gradle

// dependencies-variable.gradle
ext {
  retrofit = '2.1.0'
}

ext.dependencies = [
  retrofit2           : "com.squareup.retrofit2:retrofit:$ext.retrofit",
  retrofit2_gson      : "com.squareup.retrofit2:converter-gson:$ext.retrofit",
  retrofit2_rxjava2   : "com.jakewharton.retrofit:retrofit2-rxjava2-adapter:$ext.retrofit_rxjava2",
]

dep-7.gradle

// build.gradle
apply from :'dependencies-variable.gradle'

buildscript {
  // blah blah
}