Extend brew-test-bot to support taps

This is my stab at trying to extend brew-test-bot to support taps.

I've briefly tested this with `brew test-bot --local --tap=homebrew/homebrew-php 983d939617eec6e6b9dbd39d43f7c18f2c5e619b`, which seems to work, but I could have missed something.

I'm a ruby newbie, so be gentle ;)
`if @tap` or `unless @tap.nil?` (I'd probably prefer the prior).