Run the body of $SAFE tests on JRuby.

Apart from cases when testing whether a SecurityException is raised, run
the body of $SAFE tests without setting $SAFE.
Fix failing test cases on Rubinus and enable Travis CI rbx builds.

- Skip $SAFE tests (Rubinus doesn't support SAFE levels).
- Append an '-' argument when executing rbx as a sub-process to disable
  irb mode.
- Add to the load path and require dependencies in code rather than on
  the command line. Rubinus appears to ignore the -I and -r arguments
  when invoked with an '-' argument.
- Test whether Times are in utc using the utc? method instead of
  checking that zone returns 'UTC'. The Rubinus implementation of
  Time#zone returns 'GMT' instead of 'UTC'.