I was having this problem and it turns out there are two config.xml files. The first one is in Xcode, but you have to edit the second one from the filesystem.
AppName/config.xml
AppName/platforms/ios/AppName/config.xml
I added
<allow-navigation href="https://stackoverflow.com/questions/36234957/*" />
to both of them and it worked. Using 6.3.0.