Osx homebrew link

The Homebrew repository at GitHub now has its own user at https://github.com/Homebrew.

I also noticed that requirements_osx_brew_installation_setup() that there's a variable setting the link to its old repository, namely:

./scripts/functions/requirements/osx_brew (line 386)
homebrew_repo="mxcl/homebrew"

Later on, it's used within the requirements_osx_brew_install() function. All links work now, even though they link to the older repository. I couldn't get the https://raw.githubusercontent.com/Homebrew/homebrew/go/install working (on line 410) so I left it alone.