fix: fix running tests in IE9

static/client.html

@@ -109,7 +109,7 @@ <h1 id="title">Karma - starting</h1>
 
   <iframe id="context" src="about:blank" width="100%" height="100%"></iframe>
 
-  <script src="socket.io/socket.io.js"></script>
+  <script src="socket.io/socket.io.min.js"></script>
   <script src="karma.js"></script>
 </body>
 </html>

static/client_with_context.html

@@ -104,7 +104,7 @@
   <div id="banner" class="offline">
     <h1 id="title">Karma - starting</h1>
   </div>
-  <script src="socket.io/socket.io.js"></script>
+  <script src="socket.io/socket.io.min.js"></script>
   <script src="karma.js"></script>
   <script src="context.js"></script>
   <!-- The scripts need to be at the end of body, so that some test running frameworks

test/client/karma.conf.js

@@ -27,6 +27,13 @@ const launchers = {
     browser_version: '11.0',
     os: 'Windows',
     os_version: '10'
+  },
+  bs_ie9: {
+    base: 'BrowserStack',
+    browser: 'IE',
+    browser_version: '9.0',
+    os: 'Windows',
+    os_version: '7'
   }
 }