Not able to install upload new plugins themes

I've multi-site setup of WordPress (subdomains) 3.9.2. And I'm not able to update or install any new plugin or themes my website. Whenever I try to update any plugin the dashboard asks me FTP credentials. I dont use FTP. I use SSH for my VPS.
It's seem that something is preventing the addition in plugins or themes directory. The 'wp-content/' directory owner is apache (www-data) and has 775 permissions. I doubled checked with chown & chmod but it's still not allowing the updates/ install options via dashboard and asks FTP credentials.
When I checked, I'm successfully able to upload the media/ photos on blogs and able to edit the php files of themes or plugins via wp dashboard.
I'm stuck with old versions of plugins and themes (and entire WP itself 3.9.2 to 4.0) for few days.
Do you think it's a bug or am I missing something?