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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
status check on stagecraft to fail.

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
  - https://github.com/kennethreitz/requests/issues/749#issuecomment-19187417

These point to using this combination of libraries to replace the
default SSL handling in python2.
Merge pull request #4 from alphagov/ssl-sni-requirements

Install requirements for SSL SNI