Add gevent to test-requirements

So when we run integration tests via tox we test gevent integration as
well.

Skip gevent on pypy since travis's pypy version is too old
       Exception: PyPy 2.6.1 or higher is required

test-requirements.txt

@@ -1,3 +1,4 @@
 mock
 pytest
 pytest-cov
+gevent; "PyPy" not in platform_python_implementation