Add support for `deepseq-1.4.0.0`

This change avoids relying on `rnf`'s default method implementation
which has changed in `deepseq-1.4.0.0`

NB: previously uploaded `bytestring` releases on Hackage need retroactive
    upper bounds on `deepseq`
Merge pull request #34 from hvr/pr-deepseq-14

Add support for `deepseq-1.4.0.0`