Merge pull request #17 from eteubert/master

This fixes an error when building under Mix 1.9.0. See:

• https://github.com/ajvondrak/remote_ip/commit/a5fb55b48b999dda97aa493917e9317c4721b654#commitcomment-20730235
• https://github.com/elixir-lang/elixir/issues/9163#issuecomment-519404790

Closes #12, #14, #15, #16.