title: require_tree argument must be a directory in a Rails 5 upgraded app
body: <p>I just upgraded my app from <code>Rails 4.2.7</code> to <code>Rails 5.0.0.1</code>. I used <a href="http://railsdiff.org/4.2.7/5.0.0.1" rel="noreferrer">RailsDiff</a> to make sure I had everything covered and I believe I did. So far everything has worked well up until the loading of my app. </p>
  
  <p>Now I am seeing this error:</p>
  
  <pre><code>Sprockets::ArgumentError at /
  require_tree argument must be a directory
  </code></pre>
  
  <p>This is my <code>application.css</code>:</p>
  
  <pre><code>/*
   * This is a manifest file that'll be compiled into application.css, which will include all the files
   * listed below.
   *
   * Any CSS and SCSS file within this directory, lib/assets/stylesheets, vendor/assets/stylesheets,
   * or any plugin's vendor/assets/stylesheets directory can be referenced here using a relative path.
   *
   * You're free to add application-wide styles to this file and they'll appear at the bottom of the
   * compiled file so the styles you add here take precedence over styles defined in any other CSS/SCSS
   * files in this directory. Styles in this file should be added after the last require_* statement.
   * It is generally better to create a new file per style scope. *
   *= require_tree .
   *= require_self
   */
  </code></pre>
  
  <p>This is my <code>application.js</code></p>
  
  <pre><code>// This is a manifest file that'll be compiled into application.js, which will include all the files
  // listed below.
  //
  // Any JavaScript/Coffee file within this directory, lib/assets/javascripts, vendor/assets/javascripts,
  // or any plugin's vendor/assets/javascripts directory can be referenced here using a relative path.
  //
  // It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
  // compiled file. JavaScript code in this file should be added after the last require_* statement.
  //
  // Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
  // about supported directives.
  //
  //= require jquery
  //= require jquery_ujs
  //= require turbolinks
  //= require_tree .
  </code></pre>
  
  <p>This is what the server log looks like:</p>
  
  <pre><code>Started GET "/" for ::1 at 2016-09-02 09:08:19 -0500
    ActiveRecord::SchemaMigration Load (1.5ms)  SELECT "schema_migrations".* FROM "schema_migrations"
    User Load (1.7ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 2], ["LIMIT", 1]]
  Processing by ProfilesController#index as HTML
    Rendering profiles/index.html.erb within layouts/application
    Profile Load (1.6ms)  SELECT "profiles".* FROM "profiles"
    Rendered profiles/index.html.erb within layouts/application (45.8ms)
  Completed 500 Internal Server Error in 367ms (ActiveRecord: 6.3ms)
  
  
  DEPRECATION WARNING: #original_exception is deprecated. Use #cause instead. (called from initialize at /.rvm/gems/ruby-2.3.1@myapp/gems/better_errors-2.1.1/lib/better_errors/raised_exception.rb:7)
  DEPRECATION WARNING: #original_exception is deprecated. Use #cause instead. (called from initialize at /.rvm/gems/ruby-2.3.1myapp/gems/better_errors-2.1.1/lib/better_errors/raised_exception.rb:8)
  
  Sprockets::ArgumentError - require_tree argument must be a directory:
    sprockets (3.7.0) lib/sprockets/directive_processor.rb:182:in `rescue in block in process_directives'
    sprockets (3.7.0) lib/sprockets/directive_processor.rb:179:in `block in process_directives'
    sprockets (3.7.0) lib/sprockets/directive_processor.rb:178:in `process_directives'
  </code></pre>
  
  <p>I am using no plugins of any kind. It is a fairly simple/vanilla app. The only styling is from the default <code>scaffold.scss</code>.</p>
  
  <p>What could be causing this?</p>