Fix build on GHC >= 7.10

[rudymatela]

Lazy SmallCheck 2012 fails to build under GHC 7.10, see:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.10.3

$ cabal configure --enable-tests
Resolving dependencies...
Configuring lazysmallcheck2012-1.0.0...

$ cabal build
Building lazysmallcheck2012-1.0.0...
Preprocessing library lazysmallcheck2012-1.0.0...
[1 of 8] Compiling Test.PartialValues ( Test/PartialValues.lhs, dist/build/Test/PartialValues.o )

Test/PartialValues.lhs:149:12:
    No instance for (Applicative BT)
      arising from the superclasses of an instance declaration
    In the instance declaration for ‘Monad BT’

This seems to happen because of the Applicative-Monad Proposal.

By loosening restrictions on the cabal file, the build also fails for GHC 8.0.2.

I may get to work on this in a not-so-distant future, in that case I'll submit a PR.

[Warbo]

Just a note that there's already a pull request for this at #13 and I've been happily using this library with GHC 7.10.3 for years (from my own repo at http://chriswarbo.net/git/lazysmallcheck2012 which includes the patch). Never tried with GHC 8.x though ;)