[iOS] Try to fix the launch screen

The launch screen appears not centered and/or clipped on certain
devices. Unfortunately, I cannot reproduce it when I deploy the app to
devices and Simulator from Xcode 8.1. It may (or may very well not) be
that the Xcode version matters and the enterprise build server does
appear to use an older Xcode version.

ios/app/Base.lproj/LaunchScreen.xib

@@ -13,7 +13,7 @@
             <subviews>
                 <imageView opaque="NO" userInteractionEnabled="NO" contentMode="center" fixedFrame="YES" image="LaunchScreen" translatesAutoresizingMaskIntoConstraints="NO" id="4B8-Xf-NDE">
                     <rect key="frame" x="0.0" y="0.0" width="480" height="480"/>
-                    <autoresizingMask key="autoresizingMask" flexibleMinX="YES" flexibleMaxX="YES" flexibleMinY="YES" flexibleMaxY="YES"/>
+                    <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                 </imageView>
             </subviews>
             <color key="backgroundColor" red="0.090196078431372548" green="0.62745098039215685" blue="0.85882352941176465" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>