Metadata directory support

This separates metadata directory support from Caskfile logging as in #3066,
and will conflict with #3066.

The immediate purpose is to provide persistent per-distribution locations to for GPG data.

Also
 - incidentally adds defensive driving against null version number in method `destination_path`
 - ~~increments Travis seed due to observed error in Ruby 1.8 build~~

**Update**: the Ruby 1.8 error looks real (not a Travis glitch).  This can't be merged until that is fixed.