Use brew in homebrew_install_path/bin instead of system PATH.

Instead of using a brew that's available in system PATH, it's safer to use the one specified in homebrew_install_path/bin instead. This solves 3 scenarios:

* PATH already has a different brew that preceded the one in non-default homebrew_install_path/bin
* Using default homebrew_install_path /usr/local/, but /usr/local/bin is not available in system PATH by default
* Using non-default homebrew_install_path, with the symlink to /usr/local (as documented on README), but neither that path nor /usr/local/bin is available in system PATH

The second scenario was the problem I encountered on my Mac Mini.
I could modify the system PATH in ~/.bashrc, but I think it's more user-friendly if ansible-role-homebrew uses homebrew_install_path. It takes care of the 3 scenarios above, still compatible with the existing behaviour, and users don't have to worry about modifying system PATH.