It appears that whatever previous prevented me from using the rebar3
port_compiler is no longer an issue. This change allows rebar2 and
rebar3 compilation by relying on port_compiler for rebar3 and
reverting to standard rebar2 otherwise.

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.

Allow Jiffy to be used easily in projects using either rebar 2 or 3.

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.

Big thanks to Dmitry Groshev [1] for help getting PropEr tests into
Jiffy. These generate valid EJSON to check parsing as well as check
that random inputs don't cause segfaults or other nasty effects.

Future improvements would be to write a JSON generator as well as a
version that introduces known errors into the binary for checking known
parsing errors as well.

[1] https://github.com/si14

Fixes: #10

Passes all eep0018 tests.