Pin ooniprobe dependencies:
* Pin direct and indirect dependencies from a known working macOS
  homebrew package.
  https://github.com/Homebrew/homebrew-core/blob/master/Formula/ooniprobe.rb

Travis:
* Instruct Travis to manually fetch the gpg public key from
  deb.torproject.org as the archive signing key stored on the keyserver is
  subject to (CVE-2019-13050).
* Remove docker as it's not currenlty
* Switch to Ubuntu Xenial in Travis as Ubuntu Trusty is not a supported
  release by torproject Debian repository.
* No need for multiple version tests, dependencies pinned to a specific
  version