Fixing ownership info in .gemspec

Following [this precedent](https://github.com/alphagov/vcloud-walker/commit/a7eedfb9b88c88b76f36ac27768c4d5ab47f2c90), the .gemspec should
attribute ownership to GDS and provide a group email,
so that the team can be contacted instead of individuals
(who may or may not be maintaining this gem in the future).

/cc @annashipman @danielabel @mikepea