Bump gem to 4.0.0.
Bump gem to 4.0.0.

Because adapters are now initialised with an endpoint URL instead of a
Plek environment name, we need to bump the major gem version.

theodi/gds-api-adapters,bitzesty/gds-api-adapters,theodi/gds-api-adapters,alphagov/gds-api-adapters,bitzesty/gds-api-adapters