Unable to install any extension using magento connect manager No Answer?

Question

I have installed a fresh magento 1.9 on my vps running with nginx. My root folder of magento is /var/www/abc.hello.com/datazone/ and also set ownership as www-data as user and www-data as group (www-data nginx user). every thing goes fine while installing magento, did not show any permission error.

After installing magento, I go to admin pane system -> magento conncet -> magento connect manager, did not show any permission error on magento connect manager then i try to install "PageCache powered by Varnish" with Magento Connect 2.0 key, i paste the extension key to install, then clicked on proceed to install. In magento connect manager console at bottom it show

checking dependencies of packages
starting to download Varnish_Cache-4.0.5.tgz ...
...done: 34,730 bytes

It does not further move to installation process.

Is the problem of my installation? or permission problem ? or some think else ? I am unable to install any extension due to this big problem. I am new in magento :) Thanks in Advance.