Move CHANGELOG entry to Active Record

While the type definition is in Active Model the change of behavior will
be only user facing in Active Record so better to put the entry in its
changelog.

[ci skip]

activemodel/CHANGELOG.md

-*   Take into account UTC offset when assigning string representation of
-    timestamp with offset specified to attribute of time type.
-
-    *Andrey Novikov*
-
 ## Rails 5.0.0.beta1 (December 18, 2015) ##
 
 *   Validate multiple contexts on `valid?` and `invalid?` at once.

activerecord/CHANGELOG.md

+*   Take into account UTC offset when assigning string representation of
+    timestamp with offset specified to attribute of time type.
+
+    *Andrey Novikov*
+
 *   When calling `first` with a `limit` argument, return directly from the
     `loaded?` records if available.