getting current_command with both newer and older rebar versions

At some point rebar made `current_command` available through `rebar_config:get_xconf` instead of `rebar_config:get`. This resulted in behaviour discussed at https://github.com/basho/riak-erlang-client/issues/151 when building anything that depends on riak-erlang-client with newer rebar versions.

This commit fixes it without loosing compatibility with older rebar versions.