Declare Gradle dependencies via `implementation` instead of `compile`

compile is deprecated.

Also we should use a common form of reference to the dependency. I would opt for the second way as it's simply shorter.

compile group: 'org.apache.zookeeper', name: 'zookeeper', version: '3.6.2' // (1)
compile "khttp:khttp:1.0.0" // (2)