Pin the responses package to a working version - latest one is bugged.

I will try and raise the bug

Author: olipratt

requirements-dev.txt

@@ -4,7 +4,7 @@ coverage>=4.3.4
 Flask>=0.12
 flask-restplus>=0.9.2
 pypandoc>=1.3.3
-responses>=0.5.1
+responses==0.5.1
 sphinx>=1.5.5
 sphinx-rtd-theme>=0.2.4
 twine>=1.8.1