Cleanup for spring.io, update docs

This commit contains a number of changes, no functional difference in
the app but a general cleanup of look and docs:

- removed the Gemfile which is now redundant, Cloud Foundry CLI is no
longer a Ruby app
- updated various parts of the README to reflect changes to Spring Boot
CLI (local flag no longer exists, etc) and new CF deployment procedure.
- fixed references to www.springsource.com -> spring.io throughout web
content
- added favicon reflecting new Spring branding
- updated github URLs springsource -> spring-projects