Remove cabal.config
Remove cabal.config

I had added this in 9b8d1674510ab57a530c7aba8bba0c7dbb3b0207 in order to get
reproducible builds, before switching to Stack. But it is causing Travis CI
build failures (eg. https://travis-ci.org/wincent/docvim/jobs/137999458), so
let's remove it: Stack should give me the reproducibility I need.

wincent/docvim