lang/elixir: Update to 1.6.6

Release notes:

https://github.com/elixir-lang/elixir/releases/tag/v1.6.6

In particular, this is the first version to officially support
Erlang/OTP 21.0.

Other changes not related to the new upstream version are:

• USES= is moved before USE_GITHUB to fix a warning reported by portlint(1).