Update boostrapr

We wanted to update the bootstrapR.sh file to be more readable, by using heredoc syntax instead of echos and single-line R functions.

A significant change, however, was the reordering of the repos that are looked at as sources to install packages. Local packages, represented by `file://` were last in the list to look for, meaning that bootstrapR.sh preferred installing packages from online, even if one was present locally. 

We also made the script runnable from any location.