TypeError: $(...).selectize is not a function

I installed the "selectize-rails" gem into my rails app, and I'm trying to get it to work. I keep getting this error in my web console:

TypeError: $(...).selectize is not a function

and nothing happens in the browser. Here's the code I have so far, following the "Email Contacts" example from this page: http://ift.tt/153ib9Y

views/emails/new.html.erb

<script type="text/javascript">
  var REGEX_EMAIL = '([a-z0-9!#$%&\'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&\'*+/=?^_`{|}~-]+)*@' + '(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)';

  $('#select-to').selectize({
    persist: false,
    maxItems: null,
    valueField: 'email',
    labelField: 'name',
    searchField: ['name', 'email'],
    options: [ 
      {email: 'brian@thirdroute.com', name: 'Brian Reavis'},
      {email: 'nikola@tesla.com', name: 'Nikola Tesla'},
      {email: 'someone@gmail.com'}
    ],
    render: {
      item: function(item, escape) {
        return '<div>' +
          (item.name ? '<span class="name">' + escape(item.name) + '</span>' : '') +
          (item.email ? '<span class="email">' + escape(item.email) + '</span>' : '') +
        '</div>';
      },
      option: function(item, escape) {
        var label = item.name || item.email;
        var caption = item.name ? item.email : null;
        return '<div>' +
          '<span class="label">' + escape(label) + '</span>' +
          (caption ? '<span class="caption">' + escape(caption) + '</span>' : '') +
        '</div>';
      }
    },
    createFilter: function(input) {
      var match, regex;

      // email@address.com
      regex = new RegExp('^' + REGEX_EMAIL + '$', 'i');
      match = input.match(regex);
      if (match) return !this.options.hasOwnProperty(match[0]);

      // name <email@address.com>
      regex = new RegExp('^([^<]*)\<' + REGEX_EMAIL + '\>$', 'i');
      match = input.match(regex);
      if (match) return !this.options.hasOwnProperty(match[2]);

      return false;
    },
    create: function(input) {
      if ((new RegExp('^' + REGEX_EMAIL + '$', 'i')).test(input)) {
        return {email: input};
      }
      var match = input.match(new RegExp('^([^<]*)\<' + REGEX_EMAIL + '\>$', 'i'));
      if (match) {
        return {
          email : match[2],
          name : $.trim(match[1])
        };
      }
      alert('Invalid email address.');
      return false;
    }
  });
</script>

javascripts/application.js

//= require selectize

Does anyone who has used Selectize know what I might be missing?