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.
Merge pull request #344 from Koronen/bundle-update-sinatra-contrib

Bump `backports`, `multi_json` and `sinatra`
Add instructions to install foreman

Since pull request #336 https://github.com/swanson/stringer/pull/336 foreman is no longer part of the Gemfile. This needs installing separately as it's not covered by the bundle install (which uses the Gemfile). Finally bundle exec will no longer work on foreman as it's no longer part of the Gemfile so removed the "bundle exec" prefix for starting the application.
Merge pull request #347 from davebradford/patch-1

Add instructions to install foreman
Merge remote-tracking branch 'upstream/master'