raise AuthenticationRequiredError on Net::HTTPForbidden

This partly addresses the issue found in the larger discussion with @TimMoore 
https://github.com/bundler/bundler/issues/3180#issuecomment-59780298


This simply adds `Net::HTTPForbidden` as a `AuthenticationRequiredError`, which seems to have been inadvertently removed in this pull_request https://github.com/bundler/bundler/pull/3191

#### Before this fix
```
HTTP GET https://gem.fury.io/me/api/v1/dependencies
HTTP 403 Forbidden  # <-- broken

HTTP GET http://bundler.rubygems.org/api/v1/dependencies
HTTP 200 OK
```

#### After this fix
```
HTTP GET https://gem.fury.io/me/api/v1/dependencies
HTTP 403 Forbidden
HTTP GET https://CREDENTIALS@gem.fury.io/me/api/v1/dependencies
HTTP 200 OK  # <-- fixed

HTTP GET http://bundler.rubygems.org/api/v1/dependencies
HTTP 200 OK
```