Fix/license for cran

I'm not certain that this is the right incantation but taking a bit of inspiration from e.g. 

<http://cran.rstudio.com/web/packages/optparse/COPYRIGHTS>

I can offer this small patch to get things rolling again.
And it was not so long ago, so I am optimistic, too:
https://github.com/cran/optparse/commits/master/inst/COPYRIGHTS