* Rebar3 and erlang.mk dual support.
There was only erlang.mk support prior to this change.
The main reasons for this dual support are:
* Cover report upload can only be done by rebar3 in travis.ci
* We want to prepare for the future to build emqx releases using rebar3
* We do not want to stop supporting erlang.mk in one single step
* Add depencency version consistency check between erlang.mk and rebar.config
Gilbert