Remove Ruby 1.9.2 support (as activesupport did)

The Travis build was broken because of the activesupport 4.0.0 dependency
on Ruby 1.9.3.
https://github.com/rails/rails/blob/4-0-stable/activesupport/activesupport.gemspec#L10

Specifying conditions on gem dependencies doesn't seem to be possible.
IMHO, deprecating ruby-1.9.2 in favor of supporting rails-4.0.0 and ruby-2.0.0
is the right thing to do.