Switch phpunit install via php_pear to use :install

:upgrade action for php_pear is broken if someone is trying to set a specific version. It basically upgrades to the latest version on the first run, instead of respecting the designated version, and then all subsequent chef runs fail.

I recognize that fixing the php_pear provider would be best, but it's very convoluted and I can't quite figure it out right now in order to submit a proper PR to them.