Apple has added a tiny line over the tabBar in iOS 7 which is supposed to work as a shadow or fade between the tabBar and the UI
Swift
Nice simple solution:
Write this below code in your custom tab bar class. Then it will hide horizontal shadow line.
self.tabBar.setValue(true, forKey: "_hidesShadow")
Objective C
[self.tabBar setValue:@(YES) forKeyPath:@"_hidesShadow"];