You can't backup apps anymore. iOS 9 made that impossible.
[doublepost=1453140459][/doublepost]Sorry for the bad news. But, you can always go to the downloaded section under App Updates or in the iTunes Store to see your past downloads.
I would rather do other way around. I actually download all the apps from the iTunes and then sync with my iPhone. That way I don't have to take a back up of all the apps which I have on my phone.