Fix USE_HERMES envvar check

Summary:
Hermes is enabled by default. We set `USE_HERMES` envvar to `false` to explicilty disable Hermes. The only valid way to check it would be to compare it against `false`. Everything else will produce invalid results if `USE_HERMES` is not defined. For example `$USE_HERMES != true` will turn into `'' != true`, which evaluates to `true`.
Due to this bug source maps were not generated by default.

Changelog:
[iOS][Fixed] - USE_HERMES envvar check fixed in react-native-xcode.sh. Now source maps are generated by default.

Reviewed By: cipolleschi

Differential Revision: D38571971

fbshipit-source-id: bc6b505c225c56d62ee773b401b66d821965e106

scripts/react-native-xcode.sh

@@ -153,7 +153,7 @@ fi
   $EXTRA_ARGS \
   $EXTRA_PACKAGER_ARGS
 
-if [[ $USE_HERMES != true ]]; then
+if [[ $USE_HERMES == false ]]; then
   cp "$BUNDLE_FILE" "$DEST/"
   BUNDLE_FILE="$DEST/main.jsbundle"
 else