Install PyPyDispatcher for PyPy tests

Using https://github.com/lopuhin/pydispatcher, pypy branch.
This is executed as a separate step to avoid changing
default requirements.txt and setup.py. If just added to "deps"
in tox, this install command will be executed as one command
and PyPyDispatcher will not override PyDispatcher.

tox.ini

@@ -57,6 +57,7 @@ commands =
 [testenv:pypy]
 basepython = pypy
 commands =
+    pip install PyPyDispatcher>=2.0.6
     py.test {posargs:scrapy tests}
 
 [testenv:py33]