Upgrade to rugged 0.22.0b1

0.21.2 was just released but doesn't contain the Repository::Attributes
code we depend on. 0.22.0b1 has this code.
Merge pull request #1738 from github/update-rugged

Upgrade to rugged 0.22.0b1
Allow mime-types 2.x to be used with Linguist

The API is compatible for our purposes, and this allows Linguist to be
used in apps that pull in newer versions of mime-types through other
gems.