Prepare for v2.7.2

CHANGELOG.rdoc

+== 2.7.2
+See https://github.com/ctran/annotate_models/releases/tag/v2.7.2
+
 == 2.7.1
 See https://github.com/ctran/annotate_models/releases/tag/v2.7.1
 

lib/annotate/version.rb

 module Annotate
   def self.version
-    '2.7.1'
+    '2.7.2'
   end
 end