Fix SSL issue with phantomjs loading assets.

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

> --ssl-protocol=<val>                 Sets the SSL protocol
>   (supported protocols: 'SSLv3' (default), 'SSLv2', 'TLSv1', 'any')

This PR makes poltergeist set this option when invoking phantomjs.
Merge pull request #1781 from alphagov/fix_phantomjs_ssl_issue

Fix SSL issue with phantomjs loading assets.