internal: add gitter to broken provider list
internal: add gitter to broken provider list

Per https://developer.gitter.im/docs/authentication#2-gitter-redirects-back-to-your-site ,
both client_id and client_secret are required request parameters.

Change-Id: I64e40be0ab86ec5aea45597b6582e8059d5117cb
GitHub-Last-Rev: d1f5c9ebe6aec8e20e55d085f5e38d58146404e2
GitHub-Pull-Request: golang/oauth2#346
Reviewed-on: https://go-review.googlesource.com/c/150557
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/oauth2