Bump Cabal versions from 2.5 -> 3.0

Cabal/Cabal.cabal

Cabal/ChangeLog.md

Cabal/Makefile

Cabal/doc/conf.py

appveyor.yml

cabal-install/bootstrap.sh

cabal-install/cabal-install.cabal

cabal-install/cabal-install.cabal.pp

cabal-testsuite/cabal-testsuite.cabal

solver-benchmarks/solver-benchmarks.cabal

travis-common.sh

travis-deploy.sh

validate.sh