Make requirements_osx_brew_lib_installed_prefix_check actually read its argument

The refactoring that introduced requirements_osx_brew_lib_installed_prefix_check broke homebrew-based installation: rvm install fails with "Somehow it happened there is no executable 'openssl'…" even though brew install openssl succeeded.

The reason seems to be that requirements_osx_brew_lib_installed_prefix_check() reads $brew_lib instead of $1. Suggest replacing ${brew_lib} with $1 on this line: this lets "rvm install" finish here.
Merge pull request #2041 from jonathanperret/patch-1

Fix error locating openssl executable in homebrew-based rvm install