Add an erlang-mk target for automated updating of erlang.mk
[inaka/elvis#96] Elvis plugin, updated README.
Update Ranch, Cowlib and Cowboy versions
Merge branch 'facorro.elvis.96.plugin.readme' of git://github.com/inaka/erlang.mk
Improve c_src handling

Don't don anything if no c_src dir.
Add erl_interface headers and lib for compile and link.