Bump `backports`, `multi_json` and `sinatra`

This is the result of running `bundle update sinatra-contrib` and fixes the following error when deploying to Heroku.

> Could not detect rake tasks
> ensure you can run `$ bundle exec rake -P` against your app with no environment variables present
> and using the production group of your Gemfile.
> This may be intentional, if you expected rake tasks to be run
> cancel the build (CTRL+C) and fix the error then commit the fix:
> rake aborted!
> Bad file descriptor - bundle/ruby/2.0.0/gems/backports-3.3.5/lib/backports/1.9.1/io/open.rb
> bundle/ruby/2.0.0/gems/backports-3.3.5/lib/backports/1.9.1/io/open.rb:2:in `close'
> bundle/ruby/2.0.0/gems/backports-3.3.5/lib/backports/1.9.1/io/open.rb:2:in `open'
> bundle/ruby/2.0.0/gems/backports-3.3.5/lib/backports/1.9.1/io/open.rb:2:in `<top (required)>'
> bundle/ruby/2.0.0/gems/backports-3.3.5/lib/backports/tools.rb:328:in `require'
> bundle/ruby/2.0.0/gems/backports-3.3.5/lib/backports/tools.rb:328:in `require_with_backports'
> ...

See <https://gist.github.com/Koronen/4f95da9262f99612ed44> for the full stack trace.

Fixes #343.