Add a check for existence of the libexec modules

[Toflar]

Hey, cool project! Thank you!

I just came across this once again so I figured I'll just add this as an idea for a new feature. Depending on your setup you need to install php using e.g. brew install php70 --with-apache otherwise the module will just be missing :-) I think it would be a nice feature if the switcher checked for the existence of the module and would output something like "Are you sure you installed php via homebrew using the --with-apache command line option?". If the module does not exist, I don't think that it should be possible to switch to that version :-)