Install bundler for JRuby

Travis log:

    $ gem update bundler --no-document
    jruby: warning: unknown property jruby.cext.enabled
    Updating installed gems
    Nothing to update
    $ bundle install --jobs=3 --retry=3
    /home/travis/build.sh: line 179: bundle: command not found
    The command "eval bundle install --jobs=3 --retry=3" failed. Retrying, 2 of 3.

.travis.yml

@@ -14,7 +14,7 @@ rvm:
   - rbx-2
   - rbx
 before_install:
-  - gem update bundler --no-document
+  - gem install bundler --no-document
 before_script:
   - unset JRUBY_OPTS
 script: ruby -Ilib exe/rake