Allow installation of package 'witch' in the presence of 'hyper-switch'

I'm actually not familiar enough with homebrew to know how to test this change, but looking through the code it looks like this was the intent?  I couldn't imagine why the '$' anchor should be in the regex without the '^'.

Feel free to close this and patch yourself if you feel like this is incomplete.  For example, I didn't look in other parts of the code to see if a similar change needs to be made.

Also, while getting witch to work, I almost gave up because CONTRIBUTING.md was not updated with the awesome prefpane functionality.  Is there any other documentation that needs to be changed?