Use labs() instead of opts() to support R v3.1.0

This is @benoitc's patch from #127, I've just put together a commit and PR (assuming that he's busy!)

This fixes this error when running `make results`:

    Error: Use 'theme' instead. (Defunct; last used in version 0.9.1)
    Execution halted
    make: *** [results] Error 1

@benoitc: If you'd rather commit this yourself, please let me know!