use different review URL for iOS 7

#131 use different App Store URL template for iOS 7 since the existing one results in a blank page. Credit to @Gerharbo's commit 96bd885b16ceff985fbb79a567444053787826eb for #50 as I used that as a basis for this fix.

Note that the "fix" for iOS 6 wasn't needed a few weeks after iOS 6 was released, so if history is any indication, this fix for iOS 7 might not be needed a few weeks after the public release of iOS 7.