disable web requests, but allow localhost

I added `:allow_localhost => true` to WebMock because I was getting this error when I was trying to run my tests:

DiscourseApi::API::Topics#latest_topics returns the requested topics
     Failure/Error: topics = subject.latest_topics
     WebMock::NetConnectNotAllowedError:
       Real HTTP connections are disabled.

This fix allows localhost connections but keeps any external connections disabled. See: https://github.com/bblimke/webmock#external-requests-can-be-disabled-while-allowing-localhost