Bug 1223824 - Update WebTest from v2.0.18 to v2.0.20

http://webtest.pythonpaste.org/en/latest/changelog.html
https://github.com/Pylons/webtest/compare/2.0.18...2.0.20

requirements/dev.txt

@@ -6,8 +6,8 @@ pytest==2.8.2
 # sha256: 8UAnAQY5mPanBgl4mq6NwFcD860KNIgvYZllNlTFVUM
 datadiff==1.1.6
 
-# sha256: vrVp5tcHy0zYLCJsl7fh9BzWK65lbYnmhxB-74N2x_0
-WebTest==2.0.18
+# sha256: 5TIecoMa7o5gOKYiOHhbLWCYIz1Y-xH3FWFMnW09f9o
+WebTest==2.0.20
 
 # sha256: P1c6GL6U3ohtEZHyfBaEJ-9pPo3Pzs-VsXBXey62nLs
 mock==1.3.0