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.

This is causing serious issues for Active Admin when combined with `better_errors`: https://github.com/gregbell/active_admin/issues/2434

Other references:
+ https://bugs.ruby-lang.org/issues/6130
+ https://github.com/charliesome/better_errors/issues/134
+ https://github.com/charliesome/better_errors/issues/200