Bump Govspeak to allow/require ~> 3.0

In order to block images not hosted by us being used in Publisher items, we
need to upgrade the Govspeak version beyond what is currently allowed by
govuk_content_models.

Only constraining the major version should be safe because we follow semver:
http://semver.org
Merge pull request #212 from alphagov/bump_govspeak_to_3

Bump Govspeak to allow/require ~> 3.0.0