fix build dependencies.

There was a missing build dependency and the rules file seems to be incompatible with the other r-cran-* debian packages we have in genome-vendor. This PR fixes that.
I don't see any mention of it on our [Building Debian Packages of R Modules](https://confluence.gsc.wustl.edu/display/SYS/Building+Debian+Packages+of+R+Modules).

It is mentioned in the [changelog](http://changelogs.ubuntu.com/changelogs/pool/universe/r/r-base/r-base_3.1.1-1/changelog) for `r-base`:

>   * debian/r-cran.mk: Allow for optional $(debRreposname) to allow
    automated switching from CRAN builds to BioC builds.