I just forked the repo and using rebar I tried to build and run it.
Turns out that there's currently a new dependency on cowboy called cowlib. This means your make file in the demos directory doesn't work with the new cowlib dependency.
One way to fix this is to update line 5 in demos/Makefile to be:
-pa ../deps/*/ebin\
I guess with this change you wont need the "-pa ../deps/cowboy/ebin\ " in line 4.
Thanks
P/S: I'm a great fan. Can't stop reading everything you do