Fix btrfsprogs so nix detects the version

Currently, btrfsprogs is misdetected by nix such that it requires
specifying btrfs-progs-v3.12 for install instead of just btrfs-progs.
This patch removes the v so that it now reads btrfs-progs-3.12 and is
properly detected by nix.
Merge pull request #1421 from wkennington/btrfs

Fix btrfsprogs so nix detects the version