Erlang support on amd64

[Raphexion]

Dear Travis,

You rock and I really enjoy building on you platform.
I wonder, would it be possible to add one (or more) Erlang version for amd64.

Example
https://travis-ci.org/github/wolfpacs/wolfpacs/jobs/708334386

Example:
https://storage.googleapis.com/travis-ci-language-archives/erlang/binaries/ubuntu/16.04/aarch64/erlang-21.3-nonroot.tar.bz2

Kind regards
Niklas