Build with Elixir 1.7; Pull in CI updates from other projects

[fhunleth]

This change also deprecates support for Elixir 1.4 since we'll aren't testing with it any more.

This CI updates from other project include:

• Pulling Elixir from repo.hex.pm so that we can use a precompiled version for OTP 21
• Exercise building the hex package
• Exercise building the docs

[sourcelevel-bot[bot]]

Ebert has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).
• 1 fixed issue! 🎉

You can see more details about this review at https://ebertapp.io/github/nerves-project/nerves_uart/pulls/47.