Bump omniauth-gds to 3.1

This is necessary to ensure that auth continues to work when the token
validation in signon is tightened up. See
https://github.com/alphagov/signonotron2/pull/245 for more details.
Merge pull request #5 from alphagov/bump_omniauth-gds

Bump omniauth-gds to 3.1