Change port used for unit tests

The purpose here is to allow running guzzle3 and guzzle4 tests at the
same time, on the same machine. This is needed for
http://hhvm.com/frameworks/ to work correctly. Ideally we'll make these
more isolated in the future, but this should be an unobtrusive
short-term fix.
Merge pull request #701 from fredemmott/change-port

Change port used for unit tests