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.
Add Ruby 2.0.0 to Travis build
Add jruby-20mode to the Travis build

This commit can be reverted safely, however, since I added
Ruby 2.0.0 to the Travis build, and JRuby was supported up to
the 1.9 mode, I find quite natural to add JRuby in the 2.0 mode.