disable jruby-19mode on travis due to error on their end.

confirmed it works locally for now:

➜  binding.repl git:(master) ✗ jruby -S rake
** Invoke default (first_time)
** Invoke test (first_time)
** Execute test
Resolving dependencies...
** Execute default
Run options: --seed 56646

..................

Finished in 0.429000s, 41.9580 runs/s, 25.6410 assertions/s.

18 runs, 11 assertions, 0 failures, 0 errors, 0 skips