Add support for `deepseq-1.4.0.0`

`deepseq-1.4.0.0`'s major change is the default `rnf` method implementation
(see haskell/deepseq#1 for details).

This commit changes `time` not to rely on the default implementation and
instead explicitly make use of `seq` like the old default implementation did.
Merge pull request #11 from hvr/pr-deepseq14

Add support for `deepseq-1.4.0.0`