Set headers with Sinatra helpers

Less code!

According to docs, expires sets both `Cache-Control` and `Expires`,
taking just an integer argument:

http://rubydoc.info/gems/sinatra/Sinatra/Helpers#expires-instance_method

`content_type` is a little cleaner too:

http://rubydoc.info/gems/sinatra/Sinatra/Helpers#content_type-instance_method
Enforce semantic versioning of dependencies