Correct step 5 in Proto DataStore codelab

[remarcoble]

The last part of the code shown in the step 5 of Proto DataStore codelab does not work anymore.

Here's it's the updated one:

protobuf {
    protoc {
        artifact = 'com.google.protobuf:protoc:3.0.0'
    }

    plugins {
        lite {
            artifact = "com.google.protobuf:protoc-gen-javalite:3.0.0"
        }
    }

    generateProtoTasks {
        all().each { task ->
            task.builtins {
                remove java
            }
            task.plugins {
                lite { }
            }
        }
    }
}