pin astroid version 1.3.8 for pylint dep

requirements/edx/base.txt

@@ -150,6 +150,7 @@ django_debug_toolbar==1.5
 sqlparse>=0.2.0,<0.3.0
 
 # Used for testing
+astroid==1.3.8
 before_after==0.1.3
 bok-choy==0.5.3
 chrono==1.0.2