[installation.md] Explain how to fix homebrew to allow updates after editing formulae

Editing homebrew formulae breaks homebrew's ability to do updates. This pull request adds an explanation for how to fix the `brew update` command after editing homebrew formulae.

This is based on a [previous pull request](https://github.com/BVLC/caffe/pull/991#issuecomment-53679926) based on the master branch.