Tests on Travis don't pass anymore

[matthieudelaro]

Simple PR containing no modification resulted in red CI (#3).

After some investigation around this failing build (https://travis-ci.org/matthieudelaro/akeneo_api_client/builds/305803750), I found out that the build don't pass anymore because the underlying library responsible for issuing http requests (namely Requests) has got a new release: 2.19.

This upgrade results in different headers for each request / response, which makes vcrpy believe that played http requests are different from those recorded.