Revert "Bump Govspeak to allow/require ~> 3.0"

This is going to require more than just this change.  Reverting for now
so as to not block other changes.

This reverts commit 6403139e1e0f627d0c55957d75e446ef3389976e.
Merge pull request #214 from alphagov/revert_govspeak_bump

Revert "Bump Govspeak to allow/require ~> 3.0"