Bump gem to 5.0.0
Bump gem to 5.0.0

The changes merged with pull request #49 removed the countries method
from the content api adapter. 

As this is a breaking change, bump the gem to the next major version.
theodi/gds-api-adapters,alphagov/gds-api-adapters,theodi/gds-api-adapters,bitzesty/gds-api-adapters,bitzesty/gds-api-adapters