Support erlang.mk

[oubiwann]

There have been multiple requests for erlang.mk support, including the following PRs:

• erlang.mk compatibility #13
• Adding erlang.mk and Makefile + other little tweaks #14
• erlang.mk compatibility #15

The second two were closed as duplicates, and the first one was closed due to the original author moving on (there was a multi-year response lag from the maintainers). All the PRs offered something a little different, so whoever picks up this ticket may want to refer to them (and the associated comments) for insight.

I do not know erlang.mk, so we'll need to have some collaboration here :-)

Some notes from those PRs:

• we don't want to include erlang.mk in this project -- but!
• we can create a Makefile with a target that will download the latest erlang.mk when an erlang.mk target is run
• we'll need to update .travis.yml to add an environment for testing builds (and running tests) using erlang.mk on Travis