The recent release of Erlang 23.0 removed support for loading beam
modules compiled under R14 and earlier. Thus we're now forced to abandon
support for R13 and R14.

This prevents applications that depend on Jiffy from requiring PropEr as
a dependency just to run Jiffy's full test suite. If you want to run the
full test suite you'll have to run Jiffy's Makefile directly which
creates a `.jiffy.dev` marker that enables the full test suite.

* Update rebar to a more recent version
  * Update rebar.config for deprecation warnings
  * Allow people to export the REBAR environment variable to use
    their own version of rebar
  * Fixed `make clean`

Passes all eep0018 tests.