Rails 5 compatability

[samknight]

Rack gem version dependency needs to be upgraded to allow Rails 5 support

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    rails (< 5.1, >= 5.0.0.rc1) was resolved to 5.0.0.rc1, which depends on
      actioncable (= 5.0.0.rc1) was resolved to 5.0.0.rc1, which depends on
        actionpack (= 5.0.0.rc1) was resolved to 5.0.0.rc1, which depends on
          rack (~> 2.x)

    refile-s3 was resolved to 0.1.0, which depends on
      refile (~> 0.5.0) was resolved to 0.5.0, which depends on
        sinatra (~> 1.4.5) was resolved to 1.4.5, which depends on
          rack (~> 1.4)

[samknight]

Managed to work with the following in my gemfile

gem 'refile', github: 'refile/refile', branch: 'master'
gem 'sinatra', require: nil, github: 'sinatra/sinatra', branch: 'master'
gem 'refile-s3'