travis: Install only the GHC we are testing

[petertseng]

This will decrease install times

Closes #119

[petertseng]

The various Travis PRs #118 #120 and #128 will probably have merge conflicts with each other. I can deal with them as the PRs are merged one by one.

[petertseng]

Hmm. Adds a bit of complexity (yaml references) to the yaml file. I don't have very good measurements on how much time it saves on a build. Maybe 20 seconds per version we no longer install? That's multiplied by 2 currently, and the savings are greater if we test more GHC versions (for example 8.0 as in #120, maybe even 7.4...)

[petertseng]

will rebase in next 24 hours. I can and should be prodded if not.

[petertseng]

Does Github send notifications when I push new commits to a PR branch? I thought it did Nope, so I still have to notify by comment (let me know if I'm wrong and you in fact did get notifications). I have updated.

[rbasso]

I just received the two notifications from your messages.

But don't worry! I'm checking it frequently.

Just one more PR to go! 😄

[petertseng]

I'm just going to take a moment to say that I kept the cabal version at 1.22 across the board since this preserves existing behavior. I would note however that https://github.com/hvr/multi-ghc-travis/blob/master/make_travis_yml.hs has a map of GHC version to the cabal version at the time (well, excepting anything before GHC 7.4/cabal 1.16, but that doesn't affect us) and usually tests the two together.

I don't have a strong opinion on what cabal version should be used, so I'll leave the existing behavior untouched unless there's a compelling reason to change.