shorten MD5 hexdigests from 32 to 16 chars

See GH-130

Reduce the probability of `Errno::ENOENT` errors when the max path length limit on windows  (256 chars) is exceeded by shortening the MD5 hashes from 32 to 16 characters.

This increases the [probability of a hash collision to 1:1 billion](http://stackoverflow.com/questions/2256423/truncating-an-md5-hash-how-do-i-calculate-the-odds-of-a-collision-occuring) but should still be good enough.

Since there are two hashes involved in the cache path for a cookbook (e.g. after this PR: `tmp/librarian/cache/source/chef/site/877777683730772c/apache2/version-uri/4c32315570baa05b/package/`) this saves us 32 chars compared to master.