Set source compatibility to be Java 1.7
Set source compatibility to be Java 1.7

Accidentally was publishing 1.8 bytecode to bintray!

Fixes #7

jamescway/jruby-gradle-plugin,MisumiRize/jruby-gradle-plugin,mkristian/jruby-gradle-plugin,mkristian/jruby-gradle-plugin,MisumiRize/jruby-gradle-plugin,raelik/jruby-gradle-plugin,jamescway/jruby-gradle-plugin,raelik/jruby-gradle-plugin,jamescway/jruby-gradle-plugin,MisumiRize/jruby-gradle-plugin,raelik/jruby-gradle-plugin