Allow other Xcode configurations with DEV=false

Summary:
Updates build script to fix https://github.com/facebook/react-native/issues/4362
Closes https://github.com/facebook/react-native/pull/4520

Reviewed By: svcscm

Differential Revision: D2730607

Pulled By: mkonicek

fb-gh-sync-id: 2fc4a9815f19ab867a6a3dc77cf1b6eac8a25616

react-native-xcode.sh

@@ -14,16 +14,12 @@ case "$CONFIGURATION" in
   Debug)
     DEV=true
     ;;
-  Release)
-    DEV=false
-    ;;
   "")
     echo "$0 must be invoked by Xcode"
     exit 1
     ;;
   *)
-    echo "Unsupported value of \$CONFIGURATION=$CONFIGURATION"
-    exit 1
+    DEV=false
     ;;
 esac