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
* haskell/cabal#6342
* haskell/cabal#5888
* The full behaviour of --lib seems to not even be decided yet:
  haskell/cabal#5559

*sigh*

.travis.yml

@@ -34,7 +34,7 @@ before_install:
 - travis_retry curl -L https://www.stackage.org/stack/linux-x86_64 | tar xz --wildcards --strip-components=1 -C ~/.local/bin '*/stack'
 - stack config set system-ghc --global true
 - stack --version
-- cabal install leancheck express cmdargs || cabal install leancheck express cmdargs --lib
+- cabal install leancheck express cmdargs || cabal install leancheck express cmdargs --lib --upgrade-dependencies
 
 script:
 - make && make test