Executable doesn't resolve relative paths

Seems to be failing to resolve relative paths to files:

```
~/gh/mapbox/mapbox-upload-validate 》MapboxAccessToken=xxx bin/mapbox-upload-validate.js ./test/fixtures/valid.tm2z 

stream.js:94
      throw er; // Unhandled stream error in pipe.
            ^
Error: ENOENT, open '/test/fixtures/valid.tm2z'
```
