Update to Elixir 1.0 compatibility

Several changes were made to Elixir since this project was last updated,
leading the tests/functionality to break.

- Octal tokenization changed from leading 0 to leading 0o
- System.cmd/1 deprecated in favor of System.cmd/3
Merge branch 'elixir_1.0' of https://github.com/hderms/spawndir into elixir_1.0