Require password confirmation not to be nil on a new record

Change Validatable module to validate password confirmation presence
when creating a new record.
Previously only matching validation was being performed and the
following would work:

`User.create!(:email => 'johndoe@gmail.com', :password => '12345678', :password_confirmation => nil)`

Signed-off-by: João Soares <jsoaresgeral@gmail.com>