Drop deprecated Gem::Specification#has_rdoc= (no replacement) method

Submitted by Olle Jonsson.
diff --git a/CHANGELOG b/CHANGELOG
index 2aaa819..8a4045e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -12,6 +12,7 @@
           on the classpath. This is particularly relevant for GWT based projects that include artifacts with
           source embedded in the artifacts. This change also made it possible to remove an ugly hack in the
           GWT addon that removed the gwt artifacts from the javadoc path.
+* Change: Drop deprecated Gem::Specification#has_rdoc= (no replacement) method. Submitted by Olle Jonsson.
 
 1.5.6 (2018-05-10)
 * Fixed:  Ensure that the username and passwords declared for repositories are correctly url encoded for