reset-cache flag no longer needs boolean

Summary:
When compiling 0.31-rc1 was having issues with xcode running the package script and failing (ie not running and returning the --help info) Was due to this - simply changing the flag to --reset-cache instead of --reset-cache true worked miracles.

**Test plan (required)**

Ran packager without and runs but doesn't build the package, runs with small change and works now. Can copy paste output if you want but it's pretty super verbose for this small change.
Closes https://github.com/facebook/react-native/pull/9177

Differential Revision: D3661831

fbshipit-source-id: 3cebc543806b8fe3e413f83c59c9fb74e5e078f4

react-native-xcode.sh

@@ -84,6 +84,6 @@ $NODE_BINARY "$REACT_NATIVE_DIR/local-cli/cli.js" bundle \
   --entry-file "$ENTRY_FILE" \
   --platform ios \
   --dev $DEV \
-  --reset-cache true \
+  --reset-cache \
   --bundle-output "$DEST/main.jsbundle" \
   --assets-dest "$DEST"