move a few things around for improved cache

- separated the non-essential bits from the essential bits so that the former can be cached completely
- added an explicit pinned version for the packages from `src:r-base` so there's a natural cache-bust when it's time to update
- moved several blocks higher in the file so they can be cached completely as well
- added a default command, so that if someone does `docker run -it --rm r-base`, they're dropped straight into a useful R shell

This is more of what @yosifkit was trying to say in https://github.com/docker-library/official-images/pull/310#issuecomment-62637859 (specifically item 3 there).  I figured a pull request would make it easier to understand, since then it's a concrete proposal. :+1:

In the case of the official images, we treat them more like Debian packaging than Automated Builds are -- ie, an explicit upstream version, etc.  Hopefully that helps make this make more sense?  Happy to try to explain more if not. :smile: