Laravel Composer Update Error - Permission Denied

I'm having error during updating composer. It's a Laravel 4 project which I've downloaded from .... and first time I updated it, it was fine and got updated. but due some reason I deleted that replace it with a fresh copy of that project, but now when I started to update it, it gives me the following error:

[ErrorException]                                                           
copy(/home/username/.composer/cache/files/symfony/security-core/3a27d7b34ee6  
2cb0fdf5ad970e7777912ef4722f.zip): failed to open stream: Permission denied

In between first and second update of this composer I did the following:

1. Tried to installed Vagrant and homestead, but failed due to some internet connection problem.
2. Updated Composer failed
3. Uninstalled vagrant and reinstall composer.
4. Reinstalled Composer
5. Updated composer (success)
6. Tried to install vagrant and homestead (success)
7. Update composer (failed)
8. uninstalled vagrant
9. Reinstalled Composer
10. Update Composer (failed)

The above mentioned error is getting occured again and again. I also tried 'composer clearcache' but doesn't worked. One thing which I noticed that it's been 16-19 months I'm using composer and never heard of such errors, but since I tried to install vagrant it created hurdles and errors for me.

• I'm using Ubuntu 15.04
• Updating Laravel 4.0 project
• Updating Laravel 5

both project got updated before installing vagrant and homestead, but not now. /var/www is under the group of 'www-data' and 'username' also the part of this group.

Any help plz!!!!!!!!!!!!!!!

via Chebli Mohamed