Adopt to the recent erlang-oauth (1.3+)

Function oauth:signature/5 was removed long before erlang-oauth 1.3 but CouchDB still using this old API. Let's fix it and use oauth:sign/6 instead.

See:

* https://github.com/tim/erlang-oauth/commit/fc5f528 (pre-1.3.x changeset)
* https://github.com/apache/couchdb/blob/master/src/erlang-oauth/oauth.erl#L68