Add ElixirSips to Learning Resources

@josevalim suggested I could send in a PR adding ElixirSips to the Learning Resources in the sidebar.  For the record, this is the cover image I used:

![ElixirSips Cover](http://elixirsips.com/images/ElixirLangAd_194x232.png)

It's a modification of the image I've got on the RubyRogues site.  If it doesn't look good enough, let me know :)