Revert Travis to correct JRuby version

Currently Travis runs the JRuby with JRuby version 1.7.3, allthough version 1.7.4 is the current version.

Because Nokogiri 1.6.0 doesn't install correctly on JRuby 1.7.3 I added the 1.7.4 flag (`jruby-19mode-1.7.4`) to the `.travis.yml`.

It needs to be reverted back to `jruby-19mode` after Travis runs on JRuby 1.7.4 by default.

See [this issue](https://github.com/travis-ci/travis-ci/issues/1153) for details.