Fixed Issue with RSpec Version Check

While the original spec passed, the generator did not work in practice
because RSpec is typically not required in full in the development
environment. The generator now explicitly requires RSpec::Version to
perform the check.

lib/generators/rspec/templates/decorator_spec.rb

@@ -1,4 +1,5 @@
-<% if  RSpec::Core::Version::STRING.match(/\A3\.[^10]/) %>
+<% require  'rspec/version' %>
+<% if  RSpec::Version::STRING.match(/\A3\.[^10]/) %>
   require 'rails_helper'
 <% else  %>
   require 'spec_helper'