Added new Rscript code

I recently committed two R scripts to GitHub (https://github.com/standage/tk-rnaseq/commit/ac51150a362cb2c4cdb5c30c2b45883834ee6b31) : one was classified as Perl and the other as JavaScript. One of the R samples uses the same approach as I have (no file extension, `/usr/bin/env Rscript` on the shebang) and yet it's wrong.

I don't have much Ruby experience and was unable to get tests running locally, so I am submitting this pull request to see the test results on Travis.