Add phantomjs SSL options

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 and the ```rake test:javascript``` task set this option when invoking phantomjs.
Merge pull request #684 from alphagov/add_phantomjs_ssl_options

Add phantomjs SSL options