[racket-dev] package-system update

The latest snapshot page includes plain-source options:

  http://www.cs.utah.edu/plt/snapshots/


The reason for having both "Minimal Racket | Source" and "Minimal
Racket | Source + built packages" isn't obvious, given that Minimal
Racket doesn't include any packages --- but there is a difference (and
some future version of the web page will explain):

 * The "+ build package" variant includes pre-built core collections
   matching the pre-built packages. When you use `raco pkg install
   gui', for example, the bytecode and documentation in the downloaded
   package can be used without rebuilding.

 * The plain-source version includes no pre-built bytecode, and when
   you build it yourself, the result will be different than the one
   used to create pre-built packages (i.e., the bytecode files vary due
   to effects like `eq?'-hashing). Consequently, when you use `raco pkg
   install gui', then the bytecode and documentation of the "gui"
   package will be rebuilt, even though you're using a catalog server
   that provides built packages.