Fix add_content_id_to_orgs migration

With the migration added in ba5a421 there was both a `null:false` and
index on the new `content_id` column. This prevented the migration from
running as when it came to add the index all the rows in the column were
blank strings.

This commit changes the existing migration to add the `content_id`
column without the null constraint and updates the data migration to
look for Orgs with nil `content_id` fields instead of empty strings.
Add two organisation screenshots.
Merge pull request #1778 from alphagov/fix-content-id-migration

Fix add_content_id_to_orgs migration
Merge pull request #1780 from alphagov/add_screenshots

Add two organisation screenshots.
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.