Godeps: bump peterbourgon/diskv dependency

This will allow us to take advantage of several new features like
`Import` (eliminating a copy from cas) and a "no cache" mode (to prevent
OOMs), as well as various bug fixes.
Merge pull request #278 from jonboulle/master

Godeps: bump peterbourgon/diskv dependency
cas: never cache values in memory
Merge pull request #279 from jonboulle/master

cas: never cache values in memory