Make it compatible with RSpec 3.

From the release notes of RSpec 3:

> Remove `RSpec::Core::ExampleGroup#example` and `RSpec::Core::ExampleGroup#running_example` methods. If you need access to the example (e.g. to get its metadata), use a block arg instead
Merge pull request #75 from noniq/fix-for-rspec3

Make it compatible with RSpec 3.