Update gems and add execjs and therubyracer

Update gems, switch redcarpet to gem reference now that 3.1 is released

Update gems - merged from Robert.

All of its contents are now hosted in the wiki.

Update github-linguist, middleman, add Gemnasium status badge

You can now use <aside> with the classes

- warning
- success
- notice

to add little colored notes to your API docs! Examples are in
source/index.md.

Also, note that if you have a custom variables.scss, you'll have
to add

+$aside-notice-bg: #8fbcd4;
+$aside-warning-bg: #c97a7e;
+$aside-success-bg: #6ac174;

to it.

This bug occured if the final code sample extended beyond the
explanation at the bottom of the page.

To fix it, I simply made the background color of body a soft
blue.

Now, Slate will use localStorage to track the last selected
language, so if the user visits again with no language in the URL,
they will be redirected to the last language they used.

This update also fixes a bug with how the layout inserts the
language names into the javascript. Previously, they would be,
(for some inane reason) arrays within arrays. It is now just
an array. Whoever wrote that previous code was clearly a fool.

Also changed README wording slightly.