Clean up after Rails upgrade

1. Remove vendor/plugins since it's deprecated in Rails 4.
2. Remove strong_parameters initializer. As Rails 4 includes strong_parametes,`ActiveModel::ForbiddenAttributesProtection` is include in all AR::Base models.
3. Correct `Bundler.require`
4. Remove needless images from app/assets/images
5. Remove broken conditional comment from layout:
  1. It produces invalid markup:
  ```
  <!--[if IE 9]>
  <html class="lt-ie10" lang="en"></html>
  <![endif]-->
  ```
  2. It's not required. Foundation doesn't use this class at all.
  If you need any CSS fixes for IE8-9 then create separate stylesheet with fixes (`ie8.css`, `ie9.css`) and add it to layout in conditional comments. In that way when the time comes and you stop supporting IE8/9 you will simply remove this files and corresponding conditional comment.