requirements-dev: Fix version of mock to 2.0.0

Signed-off-by: Christopher Crone <[email protected]>

requirements-dev.txt

@@ -1,5 +1,5 @@
 coverage==4.4.2
 flake8==3.5.0
-mock>=1.0.1
+mock==2.0.0
 pytest==3.6.3
 pytest-cov==2.5.1