Fix example to work without error

[takahashim]

Fix #851.

• load grape-swagger-entity gem in :development environment
• require gem files with Bundler.require

I have a few concerns about this fix.

• We can remove other requires in config.ru such as require 'rack/cors'. Should I remove them?
• Instead of adding the grape-swagger-entity gem in :development to the common Gemfile, should I add another Gemfile for example as example/Gemfile ?

[LeFnord]

Hi @takahashim … thanks for it, to your concerns:

1. it would be nice
2. think it is not needed, cause it is for development, the gem itself doesn't require it

[takahashim]

Thank you! Fixed it and resolved the conflict.

[LeFnord]

thanks @takahashim