Support for Racket

Racket is available in the PL list when creating a repository but is not when creating a snippet.

The syntax highlighting when looking at a racket file in a repository is incorrect (not the same as on pygments.org).

Example:
https://bitbucket.org/Tetsumi/dailyprogrammer/src/master/348_easy/program.rkt\\
vs.
http://pygments.org/demo/6731097/