R: Add better package manager class

#5 (closed) changed RScriptRequirement (and others) to raise exception when version is provided, as it doesnt know how to handle a version.

It is possible to fix RScriptRequirement (#RScriptRequirement) , however that is still a very raw approach to installing R packages, and using URLs to express versions is not ideal (but that is also to Go approach).

We should use a better package manager.

https://github.com/r-lib/remotes is slightly more powerful approach, which allows versions, and this is supported by Travis CI.

https://rstudio.github.io/packrat/ is even more advanced, but needs more analysis.