Remove browser gem direct dependency

While the browser gem is great, we don't need it in this case for such a
simple usage.

There are a few really small differences between this code and the old
one: matching `/MSIE/` will return true for Opera 12 and false for
certain versions of IE11. Since we're only rendering a comment for IE8
and below, we don't care about IE11, and Opera 12 is six years old and
its users won't be affected by the comment.

Note we're still using the browser gem because ahoy_matey depends on it,
but now it's an indirect dependency.

app/views/layouts/application.html.erb

@@ -21,7 +21,7 @@
     <div class="wrapper <%= yield (:wrapper_class) %>">
       <%= render "layouts/header", with_subnavigation: true %>
 
-      <% if browser.ie? && cookies["ie_alert_closed"] != "true" %>
+      <% if request.headers["User-Agent"] =~ /MSIE/ && cookies["ie_alert_closed"] != "true" %>
       <!--[if lt IE 9]>
         <div data-alert class="callout primary ie-callout" data-closable>
           <button class="close-button ie-callout-close-js"