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
Remove old homepage images, optimise current ones

The old images don't seem to be being used by anything and just add to
confusion when looking at the folder.

Shrink the vehicle-tax image down to the largest size it will be needed
at so the homepage loads faster.
Update homepage promo to warm this winter

Update the world slot on the homepage to the warm this winter campaign.
Merge pull request #685 from alphagov/update-homepage-image

Update homepage promo. For deploy 20th October.