Add changelog entry for rspec#1856

Author: Sam Phippen

Changelog.md

@@ -10,6 +10,7 @@ Enhancements:
 
 * Allow `be_a_new(...).with(...)` matcher to accept matchers for
   attribute values. (Britni Alexander, #1811)
+* Only configure RSpec Mocks if it is fully loaded. (James Adam, #1856)
 
 ### 3.6.0 / 2017-05-04
 [Full Changelog](http://github.com/rspec/rspec-rails/compare/v3.6.0.beta2...v3.6.0)