travis: use --upgrade-dependencies
travis: use --upgrade-dependencies

... to hopefully make cabal happy

Cabal now defaults to keeping the installed version:

cf:

* https://travis-ci.org/github/rudymatela/speculate/jobs/767029407#L296
* https://github.com/haskell/cabal/issues/6342
* https://github.com/haskell/cabal/issues/5888
* The full behaviour of --lib seems to not even be decided yet:
  https://github.com/haskell/cabal/issues/5559

*sigh*

rudymatela/speculate