WordPress needs the FTP credentials to update plugins

I host wordpress on AWS EC2 (ubuntu) and encounter the following error while updating plugins:

To perform the requested action, WordPress needs to access your web server. Please enter your FTP credentials to proceed. If you do not remember your credentials, you should contact your web host.

rwx permission has been granted to the user www-data. Here is what I do.

#add a new group
groupadd www-pub  

#add the user `www-data` to the group 'www-pub'
usermod -a -G www-pub www-data 

#change the owership of '/var/www/' to 'ubuntu:www-pub'
chown -R ubuntu:www-pub /var/www 

#Change the permissions of all the folders to 2775
find /var/www -type d -exec chmod 2775 {} +

#Change the permissions of all the files to 0664
find /var/www -type f -exec chmod 0664 {} +

As you can see, www-data has all right permission but I am still required to enter the FTP credentials. Please let me know the reason and how to fix it.