don’t depned passenger formula

I use linuxbrew and nginx-full in my server,
but nginx-full is dependency the passenger formula.

I think it shouldn't be restricted only use brew to install passenger.
Many people are using rbenv to manage ruby versions.
They will be installed passenger by `gem install passenger`.

And the current nginx formula, already support find passenger-config in your path.
Just need to remove `depends_on 'passenger'`