Update to Racket 5.3.6

Racket 5.3.6

This is a bug-fix release.  It eliminates errors from v5.3.5 that
people have found over the summer.

Racket 5.3.5

This is a special-purpose release to match the arrival of "Realm of
Racket" in bookstores.  Racket v.5.3.5 adds a single `realm'
collection to the v5.3.4 release.  The new collection contains the
source code that readers of Realm may wish to use for experiments.

Racket 5.3.4

* Extflonums (80-bit floating-point numbers) are supported on some
  x86/x86_64 platforms -- including Windows, and including platforms
  where Racket is compiled to use SSE instructions for flonum
  arithmetic.  Thanks to Michael Filonenko.

* OS X: DrRacket and all of the other apps are now signed with an
  official key.

* Tally Maze: a new game based an enumeration of 2d mazes.

* The Optimization Coach, a DrRacket plugin, has been moved from the
  Racket distribution to the Racket package repository.  Install it
  with: raco pkg install optimization-coach

* Redex: `define-union-language' now merges productions when
  languages define the same nonterminals.  Thanks to William Bowman.

* The `srfi/19' library is now compatible with the date structure
  type exported by `racket/base'.

(asau)