Open R processes with --vanilla

I'm not sure if there's ever a situation where RStudio wants to open an R process _not_ using `--vanilla` -- if so, this should probably be added as a parameter, but I do think the most sensible default is always launching as `--vanilla`.

@jmcphers and I bumped into this with testing out packrat bootstrapping -- it turns out that R processes started to install `manipulate` and `rstudio` were also running the `.Rprofile` in the current working directory, which is rather undesirable.