Update dependencies chapter

[matt-dray]

I've expanded chapter 11 (dependencies) from a stub to something more substantial. It talks through the theory and provides some approaches from merely recording session information, to package managers, to docker. I think this is good enough for now and we can iterate. Note that these commits are updates to the Rmd only; I've not re-knitted the book due to packrat problems (oh, the irony).

[nacnudus]

This has been extensively reviewed by many people from the #rap_collaboration channel on the govdatascience slack and on Google docs. Once it is up, it will begin to benefit people, and will be easier to iterate.