`master` breaks build with GHC 7.6

[andreasabel]

The released clock-0.8.2 build with GHC 7.6, but not master:

$ cabal build -w ghc-7.6.3
...
Building library for clock-0.8.2..

System/Clock.hsc:23:8:
    Could not find module `Data.Coerce'

[andreasabel]

import Data.Coerce was added in PRs

• "Seconds" RealFrac instance #68
• Fix/extend tests, move Seconds into separate module #69

This module is new in base-4.7, so the constraints on base should have been updated as well.