make `Element#inspect` behave correctly in Ruby 2.0
make `Element#inspect` behave correctly in Ruby 2.0

As of ruby/ruby@fd7dc23d281f, `Kernel#inspect` no longer uses `to_s` if it's defined.
This means that if `Element#inspect` is called, you'll currently get every instance
variable printed out instead of the expected HTML.

activeadmin/arbre,escobara/arbre,activeadmin/arbre,escobara/arbre