Use Minitest 5

Hi,

this is a first attempt to use Minitest 5 for the fakeweb's test suite. I needed to do this patch to fix the build for Fedora rawhide which uses new Rails (4.1) and therefore new Minitest (5). Also, Ruby upstream already dropped `test/unit` from Ruby core so I believe it's a right time to consider using Minitest 5. I understand that you prefer to be compatible with older Rubies and Rubinius and I didn't check for the possible issues yet, but I am filing this to keep track and discuss the change. Thanks.