BLD: Move statsmodels from dev requirements into main requirements.

Pandas website states statsmodels is needed for parts of pandas.stats.
Also, update statsmodels to 0.5.0.

Upgrade of version is to both keep up to date, and to improve
performance on lazy imports, as fixed by
https://github.com/statsmodels/statsmodels/issues/573

etc/requirements.txt

@@ -6,6 +6,9 @@ Logbook==0.6.0
 pytz==2013.9
 numpy==1.8.0
 
+# Needed for parts of pandas.stats
+statsmodels==0.5.0
+
 pandas==0.12.0
 python-dateutil==2.2
 six==1.5.2

etc/requirements_dev.txt

@@ -25,6 +25,5 @@ matplotlib==1.3.1
 tornado==3.2
 pyparsing==2.0.1
 patsy==0.2.1
-statsmodels==0.4.3
 
 Markdown==2.3.1