snowplow-python-tracker: restrict test-dependencies' versions

[chuwy]

I just ran into bug where run-tests.sh fails 1 out 5 times with very strange error:

...
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/vendored_packages/pluggy.py", line 249, in _wrapped_call
INTERNALERROR>     wrap_controller.send(call_outcome)
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/capture.py", line 114, in pytest_make_collect_report
INTERNALERROR>     rep = outcome.get_result()
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/vendored_packages/pluggy.py", line 278, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/vendored_packages/pluggy.py", line 264, in __init__
INTERNALERROR>     self.result = func()
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/vendored_packages/pluggy.py", line 596, in execute
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/runner.py", line 346, in pytest_make_collect_report
INTERNALERROR>     errorinfo = collector.repr_failure(call.excinfo)
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/main.py", line 458, in repr_failure
INTERNALERROR>     return self._repr_failure_py(excinfo, style="short")
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/main.py", line 435, in _repr_failure_py
INTERNALERROR>     style=style, tbfilter=tbfilter)
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/_code/code.py", line 420, in getrepr
INTERNALERROR>     return fmt.repr_excinfo(self)
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/_code/code.py", line 632, in repr_excinfo
INTERNALERROR>     excinfo = ExceptionInfo((type(e), e, e.__traceback__))
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/_code/code.py", line 370, in __init__
INTERNALERROR>     self.traceback = _pytest._code.Traceback(self.tb, excinfo=self)
INTERNALERROR>   File "/home/vagrant/snowplow-python-3.3-tracker-environment/lib/python3.3/site-packages/_pytest/_code/code.py", line 267, in __init__
INTERNALERROR>     list.__init__(self, tb)
INTERNALERROR> TypeError: 'NoneType' object is not iterable

I believe this is pytest bug and we didn't encounter it earlier because vagrant boxes always have have fresh pytest versions.