Simplify STDERR text check in flags.feature

I ran into a failing flags.feature behat test:

https://github.com/wp-cli/wp-cli/blob/master/features/flags.feature#L43 my Behat tests fail.

Because my error says `Notice` and not `PHP Notice`.
Also there is an extra space less after `Notice:  ` while mine has only one.

I've got php 5.4.17 built-in with OSX currently. Will be switching to a different custom install because, but still. Weird that the Notice notices are different.