Workaround Travis failures (cabal-install-2.0 is broken)

[phadej]

See #5120

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

[23Skidoo]

Sure, if this fixes the issue.

[phadej]

Nope. happy has setup-depends: Cabal <2.1 for e.g. happy-1.19.8, so we see:

$ cabal new-build -j2 cabal-install:cabal
In order, the following will be built (use -v for more details):
 - Cabal-2.0.1.1 (lib) (requires download & build)
 - hackage-security-0.5.2.2 (lib) (requires build)
 - happy-1.19.8 (exe:happy) (requires download & build)

which is otherwise incorrect. @hvr proposed that we hardcode into cabal which Cabal know about which GHC and add these (upper) bounds to custom setup-depends.

[23Skidoo]

Closing in favour of #5132.