Prepare for v2.7.0

CHANGELOG.rdoc

+== 2.7.0
+See https://github.com/ctran/annotate_models/releases/tag/v2.7.0
+
 == 2.6.9
 * Support foreigh key (#241)
 * Check if model has skip tag in annotate_model_file (#167)

lib/annotate/version.rb

 module Annotate
   def self.version
-    '2.6.10'
+    '2.7.0'
   end
 end