Update to use beta version of Appraisal
Add Rails as development dependency
Add integration tests to avoid future regression

These integration tests will test middleware injection and redirection.
Update to support Rails 4.0

* Middleware now takes Sprockets environment object instead
* Removes `:digest` option support
* Removes `:manifest` setting support
Don't `bundle install --local`

Unfortunately, unless we want to list all the gems that Rails puts into
application's Gemfile by default, we can't use `--local` flag.
Update supported Rubies

* Remove 1.8.*
* Adds 2.1.0