Fix GitbookIO#1267: don't fail when "node_modules" folder doesn't exist

Author: SamyPesse

lib/plugins/findInstalled.js

@@ -63,6 +63,9 @@ function findInstalled(folder) {
 
     // List all folders in node_modules
     return fs.readdir(node_modules)
+    .fail(function() {
+        return Promise([]);
+    })
     .then(function(modules) {
         return Promise.serie(modules, function(module) {
             // Not a gitbook-plugin