Erlang 25.0 removes the ability to load modules compiled prior to Erlang
22.0. This patch replaces the Jiffy's `enc` based build system with the
`rebar3` port compiler plugin for Erlang 25.0 and newer.

This adds a dynamic check to see if -flto is supported by the compiler.

Co-Authored-By: JianBo He <heeejianbo@gmail.com>

PropEr broke my support for R14. Turns out that EQC Mini is quite usable
so I've just switched to that. If EQC Mini exists it will be used, if
not the test is skipped gracefully.

The environment variable approach breaks other projects' Travis-CI runs
if they include other projects that depend on a different version of
PropEr.

Fixes: #106

This patch adds initial support for decoding/encoding to/from the new
maps data type.

I'd like to thank Jihyun Yu (yjh0502) for the initial versions of this
work.

Floating point numbers are no longer encoded as a one to one mapping
of their binary representation, but as short as possible (while still
being acurate). The double-conversion library [1] is used to do the
hard work.

The ECMAScript compatible conversion is used.

[1] https://code.google.com/p/double-conversion/

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.