Fix constant warnings in Ruby 2.0.0 and above
Fix constant warnings in Ruby 2.0.0 and above

Since some of our projects upgraded to Ruby 2.x we've started seeing
warnings like:

```
/home/deploy/project/shared/bundle/ruby/2.0.0/gems/gds-api-adapters-7.20.0/lib/gds_api/response.rb:27: warning: already initialized constant GdsApi::Response::WEB_URL_KEYS
/data/vhost/project/shared/bundle/ruby/2.0.0/gems/gds-api-adapters-7.20.0/lib/gds_api/response.rb:27:
warning: previous definition of WEB_URL_KEYS was here
```

in our logs. This is apparently due to Ruby 2.x's behaviour when using
`require` within a symlinked path:

https://bugs.ruby-lang.org/issues/4403

One workaround would be to turn off `$VERBOSE` logging, but that means
we'd miss out on legitimate warnings.

Given that this constant was only used in this single place (I couldn't
find it in any other projects), and contained a single value, I've
simply refactored it out.

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