Allow Rakefile to run with newer versions of Rake.
Allow Rakefile to run with newer versions of Rake.

Previously `rake` would show a deprecation warning re: rake/rdoctask and
then croak.

singy/mustache,wowteacher/mustache,evilstreak/mustache,mustache/mustache,evilstreak/mustache,thelazycamel/mustache,victormours/mustache,michael-gabenna/mustache