default.nix: Upgrade to GHC 7.8.2
Relax unix dependency

The 'unix == 2.6.*' dependency was overly restrictive. We import only
one symbol (System.Posix.Process.getProcessID) from unix, so there is no
chance of a name collision later. The only potential problem is if
'getProcessID' were removed, a highly unlikely scenario.
Bump version to 0.2.0.2