Laravel 5.4 migrate报错:Syntax error or access violation: 1071 Specified key was too long; max key length is 767 bytes (SQL: alter table `users` add unique `us ers_email_unique`(`email`))

 public function up()
    {
        Schema::create(‘users‘, function (Blueprint $table) {
            $table->increments(‘id‘);
            $table->string(‘name‘);
            $table->string(‘email‘)->unique();
            $table->string(‘password‘);
            $table->rememberToken();
            $table->timestamps();
        });
    }

use Illuminate\Support\Facades\Schema;

/**
* Bootstrap any application services.
*
* @return void
*/
public function boot()
{
   //     767/4  
   Schema::defaultStringLength(191);
}