Get rid of shortid to generate buttons ids

package.json

@@ -18,7 +18,6 @@
     "less": "2.6.0",
     "less-plugin-clean-css": "1.5.1",
     "mousetrap": "1.5.3",
-    "shortid": "2.2.6",
     "uglify-js": "2.6.1"
   },
   "scripts": {

src/js/theme/toolbar.js

 var $ = require('jquery');
-var shortid = require('shortid');
 
 var gitbook = window.gitbook;
 
 // List of created buttons
-var buttons = [];
+var buttons = [],
+// Generated Id for buttons
+    BTN_ID = 0;
+
+function generateId() {
+    return 'btn-'+(BTN_ID++);
+}
 
 // Insert a jquery element at a specific position
 function insertAt(parent, selector, index, element) {
@@ -101,7 +106,7 @@ function createButton(opts) {
         index: null,
 
         // Button id for removal
-        id: shortid.generate()
+        id: generateId()
     }, opts || {});
 
     buttons.push(opts);