BLD: statsmodels requires patsy

Since statsmodels requires patsy, and statsmodels is now in
requirements.txt instead of requirements_dev.txt, patsy also needs to
be in requirements.txt instead of requirements_dev.txt.

etc/requirements.txt

@@ -8,6 +8,7 @@ numpy==1.8.0
 
 # Needed for parts of pandas.stats
 statsmodels==0.5.0
+patsy==0.2.1
 
 pandas==0.12.0
 python-dateutil==2.2

etc/requirements_dev.txt

@@ -24,6 +24,5 @@ matplotlib==1.3.1
 # tornado and pyparsing are required by matplotlib
 tornado==3.2
 pyparsing==2.0.1
-patsy==0.2.1
 
 Markdown==2.3.1