Fix issue with phantom JS loading assets

phantomjs defaults to using SSLv3, which has now been disabled
everywhere. The protocol used can be changed with an option:

--ssl-protocol= Sets the SSL protocol
(supported protocols: 'SSLv3' (default), 'SSLv2', 'TLSv1', 'any')
This PR makes poltergeist set this option when invoking phantomjs.
Merge pull request #98 from alphagov/fix-issue-with-phantom-js-loading-assets

Fix issue with phantom JS loading assets