Use Microsoft R Open + Conda in Docker Build

@dhimmel pointed out in #15 that I may benefit from using [conda](http://conda.pydata.org/docs/mro.html) to install R. This way I can specify the version and possibly allow for easy checkpoint loading. I will need to determine if all the packages in [install.R](https://github.com/greenelab/tad_pathways/tree/master/scripts/install.R) are compatible.