Check specifically for Unicorn in rack config.ru file

Don't assume that if the Rack server is not Passenger then it must be Unicorn. There are many other Rack servers in the world (uwsgi being one example that people use a lot).

The reverse check is much more logical, i.e. check explicitly for Unicorn
Merge pull request #6142 from gdamjan/master

Check specifically for Unicorn in rack config.ru file