BLD: Add libraries needed for running examples.

Add statsmodels and patsy, since they are used by example scripts
and example scripts are now run as part of the unit test suite.

etc/requirements_dev.txt

@@ -16,3 +16,5 @@ pyandoc==0.0.1
 # Example scripts that are run during unit tests use the following:
 
 matplotlib==1.2.1
+statsmodels==0.5.0
+patsy==0.1.0