Check rustup-init.sh with Shellcheck

[dwijnand]

We should verify rustup-init.sh with https://github.com/koalaman/shellcheck

Here's a simple Travis CI setup:

script:
  - shellcheck --version
  - shellcheck rustup-init.sh || travis_terminate 1

[kinnison]

So I think this will be useful to have, yes. But even if code is syntactically valid, it might sploof, so we'd still want to try running it too. This could be a good catch of the mistake we had here though.