Support RSpec 3

The way to access a current example object has been changed in RSpec 3.

* http://myronmars.to/n/dev-blog/2013/07/the-plan-for-rspec-3#core_dsl_methods_will_yield_the_example
* https://github.com/rspec/rspec-core/blob/7d6d2ca/lib/rspec/core/example_group.rb#L513-L527
Merge pull request #30 from yujinakayama/support-rspec-3

Support RSpec 3