Move to Groovy 1.6 final

This is with very great pleasure and honor that I'm announcing the
final release of Groovy 1.6, on behalf of the Groovy development team
and SpringSource.

Obviously, 1.6 is a very important milestone for the project, with
several great new features and improvements:

• great runtime performance improvements
• multiple assignments
• optional return in if/else and try/catch blocks
• AST transformations and all the provided transformation annotations
  like @Singleton, @Lazy, @Immutable, @Delegate and friends
• the Grape module and dependency system and its @Grab transformation
• various Swing builder improvements, thanks to the Swing / Griffon
  (http://griffon.codehaus.org) team
• as well as several Swing console improvements
• the integration of JMX builder
• JSR-223 scripting engine built-in
• various metaprogramming improvements, like the EMC DSL, per-instance
  metaclasses even for POJOs, and runtime mixins