Cleaner Rakefile and 1.4.0 ready

Although I have a commit access, I'm unsure if this breaks anything and that is why I'm sending a PR. I did try it out by running the server and everything seems fine. Some of the changes:


1. Cleaned up the tasks in the rake file. 
2. The ToC that in the readme gets removed via the `clean_up` method.
3. The sinatra-book is now just a link and so, removed it from the `_includes` directory.