Adjust requirements to pytest 3.0 release

pytest-monkeyplus isn't pytest 3.0 compatible yet, so we have to
explicitly forbid pytest 3.0+ in requirements.

requirements-extra.txt

@@ -1,4 +1,4 @@
-pytest>=2.0.0
+pytest>=2.0.0,<3.0
 pytest-monkeyplus>=1.0.0
 flake8