Blacklist plain gcc formula with caveat pointing to homebrew-versions.

This is part of the proposal to deprecate 'gcc' formula in homebrew-dupes and maintain versioned formulae in homebrew-versions. Related pull requests are:

https://github.com/Homebrew/homebrew-dupes/pull/145
https://github.com/Homebrew/homebrew-versions/pull/80

As there are concerns that just remove gcc formula will surprise users, this blacklist caveat is added to help user migrate.