Support older R versions

Thanks for the neat package!

I'm not sure if this was by design or not, but I went to use your package on a slightly older version of R (3.0.x) and found it wasn't supported just by merit of the minimum requirements in your `DESCRIPTION`. I changed the minimum back to a lower version and was able to install it and use it successfully. 

Thought I'd offer this change to widen your audience a bit. Of course, users would need to install from GitHub, perhaps using devtools to get this fix.