Support GHC 8.8.1

[maoe]

This PR adds support for GHC 8.8.1 and updates .travis.yml accordingly.

There are some caveats:

• Currently glib fails to build with GHC 8.8.1 on Linux. It builds fine on macOS though. So release builds use 8.6.5. 8.8.1 on Linux is marked allow_failures for now.
• The latest gtk2hs-buildtoos fails to build with GHC 8.0 and older. I dropped the old compiler support by tightening lower version bound on base.
• Until Relax upper version bounds for Cabal and gtk/gtk3 gtk2hs/gtk-mac-integration#8 gets merged, we need allow-newer: Cabal, gtk-mac-integration:gtk to build ThreadScope on macOS. See af5535e.
• AppVeyor fails even on master (AppVeyor builds are broken on master #98). I haven't been able to figure out what's going on yet. So appveyor.yml is unchanged for now.

[maoe]

While this PR still has some issues, I believe it's better than current state of affairs. I'll merge it and fix the remaining issues in separate PRs.