Removed dependency on calling setUp

This PR removes the requirement for calling `setUp`.

Is there a reason that we don't do this?
PHPUnit always create a new instance of the testcase for each test, so you always get a new Prophet anyway.

The goal of teardown is to help a bit the PHP garbage collector by removing a reference to the Prophet, thus giving a higher chance to be garbage collected by the refcount rather than the GC (but I'm not sure this helps much as Prophecy itself has cyclic object graph IIRC).