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
These code works pretty well for me (I don't really have background image for tab bar):
[tab_main.tabBar setBackgroundImage:[[UIImage alloc] init]];
[[UITabBar appearance] setShadowImage:[[UIImage alloc] init]];
And I use these code to add a frame too:
UIColor* color_green = UIColorFromRGB(0x348e5b);
tab_main.tabBar.layer.borderWidth = 0.50;
tab_main.tabBar.layer.borderColor = color_green.CGColor;
[[UITabBar appearance] setTintColor:color_green];
Hope that helps.