Update version of requests we depend on.

We currently depend on requests 0.14.2. Since the current version on PyPI is 2.4.3, we should consider updating.

What triggered this is the Travis failure https://travis-ci.org/praekelt/vumi-go/jobs/41489295 from #868 which is caused by a bug in older versions of requests that overwrote ``six.moves`` with its own module.