bump gem to 1.0.0
bump gem to 1.0.0

This release:
- reduces reliance on the dummy implementation for testing
- introduces mocking at the HTTP level with 'webmock'
- adds 'gds-api-adapters'-style test helpers

This is an compatiblity-breaking release because now the 'Users'
class is exposed through the 'Client' class.

alphagov/gds_zendesk