Don't assume ActiveModel is available

Like ActiveRecord, it is optional and its railtie may not
have been required.

lib/rspec/rails/extensions/active_record/base.rb

@@ -24,24 +24,27 @@ class ::ActiveRecord::Base
   end
 end
 
-module ::ActiveModel::Validations
-  # Extension to enhance `should have` on AR Model instances.  Calls
-  # model.valid? in order to prepare the object's errors object. Accepts
-  # a :context option to specify the validation context.
-  #
-  # You can also use this to specify the content of the error messages.
-  #
-  # @example
-  #
-  #     model.should have(:no).errors_on(:attribute)
-  #     model.should have(1).error_on(:attribute)
-  #     model.should have(n).errors_on(:attribute)
-  #     model.should have(n).errors_on(:attribute, :context => :create)
-  #
-  #     model.errors_on(:attribute).should include("can't be blank")
-  def errors_on(attribute, options = {})
-    self.valid?(options[:context])
-    [self.errors[attribute]].flatten.compact
+if defined?(::ActiveModel)
+  module ::ActiveModel::Validations
+    # Extension to enhance `should have` on AR Model instances.  Calls
+    # model.valid? in order to prepare the object's errors object. Accepts
+    # a :context option to specify the validation context.
+    #
+    # You can also use this to specify the content of the error messages.
+    #
+    # @example
+    #
+    #     model.should have(:no).errors_on(:attribute)
+    #     model.should have(1).error_on(:attribute)
+    #     model.should have(n).errors_on(:attribute)
+    #     model.should have(n).errors_on(:attribute, :context => :create)
+    #
+    #     model.errors_on(:attribute).should include("can't be blank")
+    def errors_on(attribute, options = {})
+      self.valid?(options[:context])
+      [self.errors[attribute]].flatten.compact
+    end
+
+    alias :error_on :errors_on
   end
-  alias :error_on :errors_on
 end