Add JS runtime to development env

After bundle install I get this

```shell
stbenjam@origin ~/git/katello.org (CLI*) $ jekyll serve
/home/stbenjam/.rvm/gems/ruby-2.0.0-p576/gems/execjs-2.2.1/lib/execjs/runtimes.rb:51:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
```