allow jruby to fail on Travis because reasons

Just to mention a few:
  - using guard with JRuby is so slow, so I doubt anyone is using it
  - Kernel.system doesn't support options (and no one seemed to care)
  - rdiscount doesn't install for JRuby
  - Cucumber + Aruba + Pry + threads + popen3 together make JRuby
  unusable for anything serious
  - tests pass reliably except spawned cucumber features on JRuby
avoid doc/yard related gems on JRuby
add Terminal class
refactor and fix emacs client check (nil case )
enable Sheller to run without capturing
rework and fix Guard::UI [fixes #655]
prevent watcher_spec from calling commands