Add Gradle wrapper jar to init template

[abrooksv]

Issue #, if available:

Description of changes:
The Gradle wrapper system requires the gradle-wrapper jar to be added, else you get a ClassNotFound exception

sam build
2019-03-13 11:28:54 Building resource 'HelloWorldFunction'
2019-03-13 11:28:54 /Users/alfredbr/IdeaProjects/SamJavaGradle8/HelloWorldFunction/gradlew failed to return a version string using the '-v' option. The workflow is unable to check that the version of the JVM used is compatible with AWS Lambda.
2019-03-13 11:28:54 Running JavaGradleWorkflow:GradleBuild
Build Failed
Error: JavaGradleWorkflow:GradleBuild - Gradle Failed: Error: Could not find or load main class org.gradle.wrapper.GradleWrapperMain

Checklist:

• Write Design Document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.