What a link! Of course it's actually an Upgrade sales pitch…
It is a legitimate AVG system tray notification, not a browser pop-up.
Clicking "Learn More" actually takes you here: http://www.avg.com/us-en/shellshock?A=QXIgTD00VTlTMy00QlJJTC1HVlBVNi1BNzNNSy1DN1c2Vy1EIEk9NzctMjIxOTY3MzQ1NSBMTkc9dXMgVj0yMDE1MDUzMTUgT1NWPTYuMC42MDAyIE9TPVdWU0hYODYgU1NQPTIuMCBNSUQ9YjI4NzJhNTAzODc4N2E4ZTBmOTAwMDFmOTYxYTk1ZDYtNDBkNWNmYTE5MDFhYjFlZTlhNWMzZTAzN2ExODNmOGYyYzk1Mzk4MiBCRT0yMTE5OTI2OSBBST0wIFBSVFlQRT05MCBMVD0xIFVJRD0gQUlEPSBBQ0NJRD0w&section=A&ecid=fa-avf-ot-Shellshock-f&uiid=2015-EN-38&eid=499&iid=77-2219673455 (http://www.avg.com/us-en/shellshock?A=QXIgTD00VTlTMy00QlJJTC1HVlBVNi1BNzNNSy1DN1c2Vy1EIEk9NzctMjIxOTY3MzQ1NSBMTkc9dXMgVj0yMDE1MDUzMTUgT1NWPTYuMC42MDAyIE9TPVdWU0hYODYgU1NQPTIuMCBNSUQ9YjI4NzJhNTAzODc4N2E4ZTBmOTAwMDFmOTYxYTk1ZDYtNDBkNWNmYTE5MDFhYjFlZTlhNWMzZTAzN2ExODNmOGYyYzk1Mzk4MiBCRT0yMTE5OTI2OSBBST0wIFBSVFlQRT05MCBMVD0xIFVJRD0gQUlEPSBBQ0NJRD0w§ion=A&ecid=fa-avf-ot-Shellshock-f&uiid=2015-EN-38&eid=499&iid=77-2219673455)
AVG Swami