Avoid warnings by dropping Java 1.6 and Scrooge2.

For warning *bootstrap class path not set in conjunction with -source 1.6*, set `-source 1.7` (which requires `-target 1.7`).

For warning *your code is generated by an outdated version of scrooge-generator*, remove deprecated method calls from `ThriftStruct` that existed solely to support code generated by Scrooge2 (rather than the current version Scrooge3).