Upgrade Grails, Scribe and plugins

Hi @aiten ,
thanks for this plugin :)

This PR upgrades Grails to 2.3.7, Scribe to 1.3.6, Grails Release plugin to 3.0.1, rest-client-builder plugin to 2.0.1 and set `grails.project.dependency.resolver` to Maven.

All tests are green: https://travis-ci.org/enr/grails-oauth-scribe

Cheers,
Enrico