title: After upgrading to rails 4.1, acts-as-taggable-on is not working
body: <p>I have upgraded my rails version of one my app to 4.1.0. While upgrading I have upgraded all gems from Gemfile, which upgraded acts-as-taggable-on to 3.1.1 too.</p>
  
  <p>but after upgrade, when I tried to add tags on respective object, it is failing with following error.</p>
  
  <pre><code>  post = Post.last
    post.tag_list = "development"
    post.save
  </code></pre>
  
  <blockquote>
    <p>ERROR:  column "taggings_count" does not exist</p>
  </blockquote>
  
  <p>details error is here:</p>
  
  <pre><code>SQL (0.9ms)  UPDATE "tags" SET "taggings_count" = COALESCE("taggings_count", 0) - 1 WHERE "tags"."id" = 3146
  PG::UndefinedColumn: ERROR:  column "taggings_count" does not exist
  LINE 1: UPDATE "tags" SET "taggings_count" = COALESCE("taggings_coun...
                                                        ^
  : UPDATE "tags" SET "taggings_count" = COALESCE("taggings_count", 0) - 1 WHERE "tags"."id" = 3146
     (0.3ms)  ROLLBACK
  ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column "taggings_count" does not exist
  LINE 1: UPDATE "tags" SET "taggings_count" = COALESCE("taggings_coun...
                                                        ^
  : UPDATE "tags" SET "taggings_count" = COALESCE("taggings_count", 0) - 1 WHERE "tags"."id" = 3146
  </code></pre>