Add Travis CI build status image to README
Pin Python package versions in requirements.txt

Fixes #189

Note that I used the latest versions from PyPI on 18th August 2014.
Merge pull request #230 from paulfurley/add-travis-badge-to-readme

Add Travis CI build status image to README
Merge pull request #231 from paulfurley/189-pin-versions-in-requirements.txt

Pin Python package versions in requirements.txt