Cabal-based Travis builds cache compiled binaries

Cabal-based Travis build put `~/.cabal/bin` on the path even after though `.cabal` is cached, meaning we use whatever stack was build by the last build that updated the cache. In #2516 that's a stack with a bug due to some previous build.

In comparison, the stack-based build puts `.local` on the path, but that's not cached.

Possible solution: remove `~/.cabal/bin/stack` before starting the build. Other executables there might be needed, and they should be safe to cache.