Travis: Actually use gcc on OS X

On macOS, gcc is an alias to clang:

GCC version
Apple LLVM version 9.1.0 (clang-902.0.39.2)
Target: x86_64-apple-darwin17.4.0
Thread model: posix
InstalledDir: /Applications/Xcode-9.4.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

https://travis-ci.org/teor2345/tor/jobs/421941946#L35

Doc update: https://github.com/travis-ci/docs-travis-ci-com/pull/2002

If we want to use gcc on OS X, we have to set a specific GCC version in the environmental variables: https://docs.travis-ci.com/user/languages/c/#gcc-on-os-x

Edit: canonical URL