updated impressionist initializer

lib/generators/templates/impression.rb

@@ -3,4 +3,6 @@
   # Define ORM. Could be :active_record (default), :mongo_mapper or :mongoid
   # config.orm = :active_record
 #end
-Impressionist.orm = :<%= options.orm if options.orm %>
+
+<% orm  = options.orm.match(/mongoid|active_record|mongomapper/) %>
+<%= "Impressionist.orm = :#{options.orm}" if  String === options.orm %>