Ignore `disable_functions` from `php.ini`

For security reasons, some production environments have a long list of disabled functions.

A simple `wp` would fail when `proc_open()` is disabled.

Reminder: `--no-php-ini` is more dangerous as it will ignore `open_basedir` directives thus allowing cross-site worms to spread.
Thanks for your suggestion!

I was not sure if `=""` works in all environments, so I used the second example [here](http://php.net/manual/en/features.commandline.options.php#example-383).

Tested just now. Looks like your solution works everywhere. :D

Commit b129dcf added to this pull request.