Update README.md

Homebrew recently booted a bunch of Formulae to their science repository, VIPS has been included in the move. 

I opened up a ticket about this with Homebrew as the change has frustrated me a great deal and I think makes it substantially less easy to use.

Regardless, for the mean time, this is how to install VIPS via homebrew now.
Merge pull request #33 from tehprofessor/patch-1

Update README.md