Laravel 5.4 installation error.

I am setting up my project using laravel installer command,

laravel new blog

But it returns the following errors.

Generating autoload files

php -r "file_exists('.env') || copy('.env.example', '.env');" Class Illuminate\Foundation\ComposerScripts is not autoloadable, can not call post-install-cmd script php artisan optimize PHP Warning: require(/var/www/blog/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php): failed to open stream: No such file or directory in /var/www/blog/vendor/composer/autoload_real.php on line 55 PHP Fatal error: require(): Failed opening required '/var/www/blog/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php' (include_path='.:/usr/share/php') in /var/www/blog/vendor/composer/autoload_real.php on line 55 Script php artisan optimize handling the post-install-cmd event returned with an error

[RuntimeException]
Error Output: PHP Warning: require(/var/www/blog/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php): failed to open stream: No
such file or directory in /var/www/blog/vendor/composer/autoload_real.php on line 55
PHP Fatal error: require(): Failed opening required '/var/www/blog/vendor/laravel/framework/src/Illuminate/Foundation/helpers.php' (include
_path='.:/usr/share/php') in /var/www/blog/vendor/composer/autoload_real.php on line 55

run-script [--timeout TIMEOUT] [--dev] [--no-dev] [-l|--list] [--] [] []...

Application ready! Build something amazing.

My development environment setup:-

Ubuntu 16.04, 64 bit
Composer version 1.0.2
Laravel Installer 1.3.3
PHP 7.0.8-0ubuntu0.16.04.3

via Chebli Mohamed