Update viewer.html for Internet Explorer Compatibility

If your Internet Explorer 11 default compatibility settings are set to "I.E 7 Compatibility", the PDF plugin will not load.  This fix is the same one used by AngularJS to force the browser to use HTML5 mode.

web/viewer.html

@@ -26,6 +26,9 @@ See https://github.com/adobe-type-tools/cmap-resources
     <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
 <!--#if GENERIC || CHROME-->
     <meta name="google" content="notranslate">
+<!--#endif-->
+<!--#if GENERIC-->
+    <meta http-equiv="X-UA-Compatible" content="IE=edge">
 <!--#endif-->
     <title>PDF.js viewer</title>