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.