Ask to user to untap the old PHP brew repository

[sroze]

For users using old homebrew installation, they may have tapped josegonzalez/php, so they'll have the following error:

OUT ==> Tapping homebrew/php
ERR Cloning into '/usr/local/Library/Taps/homebrew/homebrew-php'...
OUT Tapped 535 formulae (571 files, 2.6M)
RUN brew install php56-ssh2
ERR Error: Formulae found in multiple taps:
ERR * homebrew/php/php56-ssh2
ERR * josegonzalez/php/php56-ssh2
ERR Please use the fully-qualified name e.g. homebrew/php/php56-ssh2 to refer the formula.

The workaround is to brew untap josegonzalez/php. We should explicitly propose them to do so.