Remove referencesOthers toggle

The [referenceOthers](https://github.com/docpad/docpad-plugin-less/blob/master/src/less.plugin.coffee#L36) toggle is as follows:
``` coffeescript
# Add Reference Others if this document does
# As lesscss concats imports
file.setMetaDefaults('referencesOthers': true)  if opts.content.indexOf('@import') isnt -1
```

This tells DocPad to rebuild all documents if `@import` is anywhere in the document. There are two cases of using a `@import` in a LESS file:

1. Importing a LESS file. This will make LESS concatinate and re-build the LESS file for us.
2. Importing a CSS file. LESS will ignore concatenation and just leave the CSS as a CSS import.

In either case, it seems like LESS will do the work for us, and not have to rebuild anything. It seems like we don't need the referencesOthers toggle at all. I suggest we remove it, and just default to reference Others false. LESS handles this for us.