adding gunicorn to base requirements

requirements/edx/base.txt

@@ -35,6 +35,7 @@ feedparser==5.1.3
 fs==0.4.0
 GitPython==0.3.2.RC1
 glob2==0.3
+gunicorn==0.17.4
 lazy==1.1
 lxml==3.0.1
 mako==0.7.3