mapnik-omnivore update

Increases filetype support per https://github.com/mapbox/mapnik-omnivore/issues/52 and https://github.com/mapbox/mapnik-omnivore/issues/50

@rclark the [second issue above](https://github.com/mapbox/mapnik-omnivore/issues/50) might be something to consider for mapbox-file-sniff?