Prepare for v2.6.8

CHANGELOG.rdoc

+== 2.6.8
+* Nothing annotated unless options[:model_dir] is specified, #234
+
 == 2.6.7
 * Nothing annotated unless options[:model_dir] is specified, #234
 

lib/annotate/version.rb

 module Annotate
   def self.version
-    '2.6.7'
+    '2.6.8'
   end
 end