python: pin httpretty to fix tests on travis
construct==2.9.33
futures>=2.2.0
-httpretty
+httpretty==0.9.4
+# httpretty 0.9.5 is causing issues on travis: https://github.com/gabrielfalcao/HTTPretty/issues/340
pyftdi==0.13.4
pylibftdi
pyserial
