circle.yml: update python-requests pkg

Update the build-tools introduced a regression while building on circle
ci.

The PR https://github.com/weaveworks/build-tools/pull/73 is compatible
with python requests module `>=v2.4.2`. We need to provide a more
updated version of the one available in ubuntu 12.04 or 14.04
repositories.

circle.yml

@@ -16,6 +16,8 @@ machine:
     SCOPE_UI_BUILD: $HOME/docker/scope_ui_build.tar
 
 dependencies:
+  pre:
+      - pip install --upgrade requests
   cache_directories:
     - "~/docker"
   override: