Use new Racket lexer from pygments.rb 0.3.3

Racket files had been using the Scheme lexer.
Update test: Racket language uses Racket lexer.

This is https://github.com/greghendershott/linguist/pull/1 from @tnm.
That pull request is onto my master branch, not my `racket-lexer`
topic branch. If there is a way to accept the pull request onto my
topic branch, I don't have time to figure it out right now. As a
result I'm making my own commit.
Merge pull request #325 from greghendershott/racket-lexer

Use new Racket lexer from pygments.rb 0.3.3