Run npm install after project has been updated.

When initializing a new project, we should run `npm install` so that
the project directory starts off clean. When updating an old project,
this will update dependencies.

Change-Id: Idd7602875480b57ab96d5151773761650a94f58c

generators/app/index.js

@@ -106,6 +106,10 @@
       excluded.forEach(function (path) {
         self.fs.delete(self.destinationPath(path));
       });
+    },
+
+    install: function () {
+      this.npmInstall();
     }
   });
 })();