HipChat service: default to v2 API

HipChat no longer recommends people use the deprecated v1 API so switch
to using the v2 API by default. hipchat-rb does not yet default to v2 in
any version so it must be specified.
HipChat service: support custom servers

HipChat allows users to run their own private servers and to be able to
support those we must connect to the correct URL when using one of these
custom servers.