Incomplete Rakefile

ThinkingSphinx seems to require the app to define an "environment" target, which my Sinatra app did not. I hadn't been using a Rakefile, so when I made one with only "require 'thinking_sphinx/tasks'", I got a bunch of nonsense. I've posted what made it work for me; perhaps there is a more elegant or general solution.

Contributed by @andrewpcone