Remove Pod::UI undefined symbol warning during Hermes build

Summary:
The `utils/build-apple-framework.sh` script uses the ruby runtime to load `hermes-engine.podspec`, and in the process it will load the Core CocoaPods gem to parse the podspec file.

Since the podspec file uses Pod::UI to print logs, and the CoreUI gem is not loaded when `build-apple-framework.sh` accesses `hermes-engine.podspec`, we would get a warning due to Pod::UI not being defined.

This change will only use Pod::UI when it is defined, therefore limiting it to only executing when `hermes-engine.podspec` is loaded as part of the normal `pod install` build process.

Changelog: [Internal]

Reviewed By: cortinico, neildhar

Differential Revision: D36297856

fbshipit-source-id: ac1ab021f74f9dcaa7be2a2358482c73b827b756

Author: hramos

sdks/hermes-engine/hermes-engine.podspec

@@ -16,7 +16,7 @@ module HermesHelper
   BUILD_TYPE = :release
 end
 
-Pod::UI.puts '[Hermes] Hermes needs to be compiled, installing hermes-engine may take a while...'.yellow
+Pod::UI.puts '[Hermes] Hermes needs to be compiled, installing hermes-engine may take a while...'.yellow if Object.const_defined?("Pod::UI")
 
 Pod::Spec.new do |spec|
   spec.name        = "hermes-engine"