erlang: bump to version 20.0

erlang: bump to version 20.0

This change bumps the Erlang version from 19.3 to 20.0. As a new major
release, there are a few incompatibilities. Details are here:
http://www.erlang.org/news/114

This should fix builds on ppc64le host machines:
http://autobuild.buildroot.net/results/407af9800313d4540643d7625e85acc0c2366892
http://autobuild.buildroot.net/results/2aca54ad48ecfd72242d97499e21a46c1b384b10

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Reviewed-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>