Release version 1.4.0

Hey @DatabaseCleaner/owners 

We should release a new version of the gem. 

A few things that need to be done:

- [ ] Upgrade gemspec to version 1.4.0
- [ ] Tag master as v1.4.0
- [x] Update History.rdoc with the changes in https://github.com/DatabaseCleaner/database_cleaner/compare/v1.3.0...master
- [x] Upgrade copyright notice to 2014

I wanted to create this issue to make sure I don't miss anything. What do you think @JonRowe?

Thanks!