Add a check to support both RSpec 3 & 4

RSpec 4 has removed OperatorMatcher, as it was only supported by the
should syntax, and not expect syntax.

Author: pirj

lib/rspec/rails/matchers/relation_match_array.rb

@@ -1,3 +1,3 @@
-if defined?(ActiveRecord::Relation)
+if defined?(ActiveRecord::Relation) && defined?(RSpec::Matchers::BuiltIn::OperatorMatcher) # RSpec 4 removed OperatorMatcher
   RSpec::Matchers::BuiltIn::OperatorMatcher.register(ActiveRecord::Relation, '=~', RSpec::Matchers::BuiltIn::ContainExactly)
 end