Force the use of Java 6 class files

Using the plugin with an active Java 7 installation caused it to generate Java 7 class files by default. Bugs caused by this ranged from not being able to build the app (`dex` bailed out when seeing the invalid file) to simply missing parts of the app (the missing `R.class` was reported lots of times). This commit fixes that by setting the correct `scalacOptions` and `javacOptions` by default in the plugin.