Remove annoying warning on latest versions of Gradle

The groovy configuration has been deprecated and is scheduled to be removed in Gradle 2.0. Typically, usages of 'groovy' can simply be replaced with 'compile'. In some cases, it may be necessary to additionally configure the 'groovyClasspath' property of GroovyCompile and Groovydoc tasks.
Update RatpackGroovyPlugin.groovy
Just use compile configuration instead of deprecated groovy
Throw exception if unsupported Gradle version is used
Merge pull request #309 from aadamovich/patch-1

Remove annoying warning on latest versions of Gradle