BLD: Add mock to requirements file.

mock was recently added as part of the testing of port of the script
API for algorithms.

etc/requirements_dev.txt

@@ -3,6 +3,7 @@ nose==1.3.0
 nose-parameterized==0.3.3
 nose-ignore-docstring==0.2
 xlrd==0.9.2
+mock=1.0.1
 
 # Linting