plugins/hipchat-v2: support HipChat API v2

Fixes #113 (HipChat API v2 Support)

So manual testing works fine for both API versions. However, this patch
being integrated into the website, provides only suppor for API v1.
It returns 401 for API v2 keys.

Maybe it's something with my configuration. It would be nice if anyone
else could also test.
plugins/hipchat/index: fix the constant's name
plugins/hipchat/config: do not bring attention to the version
Merge pull request #120 from bugsnag/113-hipchat-api-v2-compat

113 hipchat api v2 compat