Fix react-native-xcode.sh build script

Summary:
This diff fixes USE_HERMES checks.

Changelog
[iOS][Fixed] - USE_HERMES envvar check fixed in react-native-xcode.sh.

Reviewed By: cipolleschi

Differential Revision: D38704558

fbshipit-source-id: 90a8648fd1094fcf9ab085f37efce9b811e707bc

Author: Dmitry Rykun

scripts/react-native-xcode.sh

@@ -129,7 +129,7 @@ fi
 
 PACKAGER_SOURCEMAP_FILE=
 if [[ $EMIT_SOURCEMAP == true ]]; then
-  if [[ $USE_HERMES == true ]]; then
+  if [[ $USE_HERMES != false ]]; then
     PACKAGER_SOURCEMAP_FILE="$CONFIGURATION_BUILD_DIR/$(basename $SOURCEMAP_FILE)"
   else
     PACKAGER_SOURCEMAP_FILE="$SOURCEMAP_FILE"
@@ -138,7 +138,7 @@ if [[ $EMIT_SOURCEMAP == true ]]; then
 fi
 
 # Hermes doesn't require JS minification.
-if [[ $USE_HERMES == true && $DEV == false ]]; then
+if [[ $USE_HERMES != false && $DEV == false ]]; then
   EXTRA_ARGS="$EXTRA_ARGS --minify false"
 fi