More conservative fix re: lib-R symlinks

This fix should allow multiple R sessions of the same version to use the same packrat `lib-R` directory.

This is done by ensuring pre-existing symlinks are only updated in a new version of R is detected (ie, with the session R not equal to the version of R associated with `base` in `lib-R`).

@jjallaire, can you double check that this is sensible? I think it should ensure any pre-existing packrat + RStudio machinery should play nicely together.