update pymysql to 0.6.6 to avoid cursor bugs

I managed to repeat the bug I was getting on travis with version 0.6.1

Author: Thomas Grainger

ci/requirements-2.7.txt

@@ -17,7 +17,7 @@ boto=2.36.0
 bottleneck=0.8.0
 psycopg2=2.5.2
 patsy
-pymysql=0.6.1
+pymysql=0.6.6
 html5lib=1.0b2
 beautiful-soup=4.2.1
 httplib2=0.8