require 'set' is necessary when running PP live (not in tests)
code in at_exit in invalid - fixes #8

Travis would complain :
```
/home/travis/build/alainravet/private_please/spec/../lib/private_please.rb:29: undefined local variable or method `candidates_store' for main:Object (NameError)
The command "bundle exec rspec" exited with 1.
Done. Your build exited with 1.
```