Upgrade Travis to haskell-ci

[RyanGlScott]

Currently, the Travis script is using a hacky setup that should probably be replaced with haskell-ci. I've made an effort to do so on the tests-in-separate-component branch. Because the dependencies of the test suite depend on deepseq, we have to put the tests in a separate .cabal file to work around haskell/cabal#5200, à la haskell/primitive#90 and ekmett/transformers-compat#33.

Alas, that branch currently is failing to build. It's always the same failure, when trying to build time:

cabal: can't find include file HsTimeConfig.h

I don't know how to fix this, so I'm giving up for now.

[RyanGlScott]

It looks like this is a cabal new-build–specific issue. See haskell/cabal#5223.

Until that can be resolved, it seems we're blocked here.

[RyanGlScott]

As noted on haskell/cabal#5169, a workaround for this issue is downgrading to cabal-install-2.0. That makes some of the build configurations work, but there's still issues on GHC 7.4 and 7.6. Here's an example build failure to puzzle over.

[RyanGlScott]

Rats, I think the GHC 7.4 and 7.6 build errors observed in #39 (comment) are caused by cabal-install-2.0 having a worse solver than 2.2, since with cabal-install-2.2, I'm able to successfully construct a build plan on 7.4 and 7.6. Unfortunately, I can't actually build it due to haskell/cabal#5223, so we're once again stuck on that.

[chessai]

We should just use GitHub actions now, which should work fine, especially when we drop 7.x support. Please re-open if I'm wrong.