Log Message:
Update Racket to version 5.1.3

Changes in Racket 5.1.3

This is a bugfix release, resolving the DrRacket issue with
the contour view.  In addition, two tex files with problematic
licensing were removed.

Changes in Racket 5.1.2

* The download page includes 64-bit installers for Mac OS X,
  Windows, and two Debian flavors.  Racket now supports OS X Lion.

* Racket now includes a new `racket/place' library to support
  parallelism, complementing `racket/future'.  Racket's parallel
  build process is now based on places instead of multiple OS
  processes.

  Places support share-nothing parallelism and message-passing
  communication.  Compared to futures, places are heavyweight, but
  they have a simpler performance model.

* The syntax-certificate system has been replaced by a syntax-taint
  system.  Both certificates and taints were designed to protect
  otherwise inaccessible bindings from abuse when they appear in
  macro expansions.  Taints are simpler and lighter, and the switch
  closes known holes in the certificate system.  Macros that are not
  implemented with `syntax-rules' or `define-syntax-rule', however,
  must explicitly use `syntax-protect' to protect their expansions
  from abuse.

* The `net/url' library supports HTTPS connections, but beware that
  by default all sites are accepted (equivalent to ignoring a
  browser's warnings about untrusted certificates).

* Error messages in the student languages use a simplified
  vocabulary and consistent phrasings.  If you maintain curriculum
  material or teachpacks then please consider updating.  See the
  "Error Message Composition Guidelines" section in the
  documentation for details.

* Typed Racket: almost all core Racket data structures and
  operations are now accessible in Typed Racket (most of this work
  is due to prolific contributor Eric Dobson).  The performance of
  the typechecker has been significantly improved.

* The `scriblib/bibtex' library supports BibTeX-formatted citation
  databases in Scribble documents.  BibTeX can be tricky to parse,
  so please report failed entries as bug reports.

* The `for' forms now support an `#:unless' clause, and a
  nonnegative integer can be used as a sequence.  The new `compose1'
  function creates single-valued composition functions.  The
  `racket/function' library now provides `identity', `thunk', and
  `thunk*'.

* The license has been clarified: we now use LGPLv2.1 uniformly.
  (The license file used to specify LGPLv2, contrary to the download
  pages.)