Support the Applicative-Monad proposal and depend on it

[ggreif]

Implement issue #21.

Since GHC v7.10.x Applicative a required constraint on Monad. This PR takes advantage of that fact, by bulk changing paired constraint in favour of Monad.

Needs:

• [x] bump of base dependencies to 4.8
• [x] maybe bump of varying version to 0.7.1
• [ ] maybe remove the warning suppression for redundant constraints

Possible follow-up actions:

• [ ] Issue to upgrade Travis test matrix (with newer GHCs)
• [ ] Issue to re-include varying into Stackage nigthtly (urgent, cutoff date for lts-12 near)

[schell]

I agree with all of your points. We'll definitely do a minor version bump. If you'd like to tackle the travis changes that's cool, but I've also been meaning to move to gitlab's CI. If you do decide to do the travis stuff I'm not opposed to keeping the CI there, as this lib is pretty stable these days.

[ggreif]

Feel free to merge