Install requirements for SSL SNI

We use SNI to serve up *.<env>.performance.service.gov.uk in production,
sadly python2 does not support SNI out of the box which causes our
AdminAPI to fail when connecting to Stagecraft in production.

There is information about it in:
  - http://docs.python-requests.org/en/latest/community/faq/
  - https://stackoverflow.com/questions/18578439/using-requests-with-tls-doesnt-give-sni-support/18579484#18579484
  - kennethreitz/requests#749 (comment)

These point to using this combination of libraries to replace the
default SSL handling in python2.

These are only applicable for python2 and break with 3, so we only want
to install them when we are running python < 3
Bump the version of the lib

We need to release a new version of the library that includes these new
dependencies
Merge pull request #21 from alphagov/allow-sni

Allow sni