Update commands and README to fit `brew tap`

The changes in `brew tap` had a few extra side-effects that I hadn't
thought of. These updates account for those, hopefully without breaking
backwards compatibility of `brew any-tap` itself.

Ping @englishm, @taojonesin, @mr-tim, @jmcgarr, @ncrowell, @cooljeanius, @ipmcc, @gvol and @dturner-tw

Sorry to bother, but if any of you are still using `anytap`, please feel free (encouraged?) to take a look at this or try it out via a manual installation. I want to make sure that (1) this doesn't break backward compatibility but (2) it fits the changes in `brew tap`. If you don't have time, no huge deal, but I wanted to think hard about this before pushing a change live. Thanks.