Upgrade gradle to 5.6.2

[orangain]

This PR contains the following changes. I believe they will provide better Gradle experience to generator user.

Changes

• Upgrade gradle versions to 5.6.2 in both project itself and generated project.
  • There are many improvements in Gradle 5.x including better Kotlin DSL support. https://gradle.org/whats-new/gradle-5/
• Replace deprecated compile/testCompile with implementation/testImplementation.
  • https://docs.gradle.org/current/userguide/java_plugin.html#sec:java_plugin_and_dependency_management