Fix syntax for appending elements to a collection

Gradle 2.0 upgraded Groovy to version 2.3.2.  This newer version of Groovy uses a different syntax for appending an element to a collection.  See the [release notes](http://www.gradle.org/docs/current/release-notes#+=-operator-changes-that-typically-affect-configuration-of-eclipse-and-idea-plugin).

This pull request uses the new Groovy syntax and should fix the error mentioned in johndevs/gradle-vaadin-plugin#113.  All tests are passing.