Backward support for ruby < 1.9.2

Avoid below exception in ruby 1.8

/home/vagrant/rspec-example_steps/lib/rspec/example_steps.rb:20: undefined method `singleton_class' for RSpec::Core::ExampleGroup:Class (NoMethodError)
	from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
	from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `require'
	from /home/vagrant/rspec-example_steps/spec/spec_helper.rb:1
	from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
	from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `require'
	from /home/vagrant/rspec-example_steps/spec/example_steps_spec.rb:1
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `load'
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `load_spec_files'
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `each'
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `load_spec_files'
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:22:in `run'
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:80:in `run'
	from /var/lib/gems/1.8/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:17:in `autorun'
	from /usr/local/bin/rspec:19