Tighten Safe Haskell bounds, fixes new warning in GHC 7.10.
Add support for `deepseq-1.4`

This change avoids relying on `rnf`'s default method implementation
which has changed in `deepseq-1.4.0.0`
Merge pull request #65 from dterei/safe710fixes

Tighten Safe Haskell bounds, fixes new warning in GHC 7.10.
Merge pull request #67 from hvr/pr-deepseq-14

Add support for `deepseq-1.4`