Add ConstraintKinds for GHC 7.4
Add ConstraintKinds for GHC 7.4

I don't quite understand why this is needed. This is a workaround for
https://travis-ci.org/maoe/influxdb-haskell/jobs/22959983.

alphaHeavy/influxdb-haskell,maoe/influxdb-haskell