Start testing with JRuby too.

However, because JRuby stable does not support 2.0/21 mode on Travis (reliably as far as I'm aware) we only test on JRuby-head right now because we have dropped support for any EOL Ruby and master contains some code that might or might not fail out on 1.9.

.travis.yml

@@ -5,6 +5,10 @@ rvm:
 - 2.2
 - 2.1
 - 2.0
+- jruby-head
+matrix:
+  allow_failures:
+    - rvm: jruby-head
 env:
   matrix:
     - TEST_SUITE=test