Plugin not compatible with JDK 8 and Grails 3

The plugin has issues compiling when using JDK 8 and Grails 3 and Groovy versions >=Groovy 2.3.5. 

Groovy Issues:
- ShortTypeHandling (Temp Fixed by including `compile 'org.codehaus.groovy:groovy-backports-compat23:2.3.5'` in `BuildConfig.groovy`)

Grails Issues:

Java Issues:

I will try and update this issue with other errors thrown and any known solutions I find.
