Make Hermes from source the default

Summary:
If Hermes is enabled, it will be built from source instead of using the CocoaPods hermes-engine pod.

Changelog:
[iOS] [Changed] Build Hermes from source when Hermes is used

Reviewed By: cortinico

Differential Revision: D35693945

fbshipit-source-id: eadc638106180b183fd6d1e2a272dc3df66fa8b6

scripts/react_native_pods.rb

@@ -110,14 +110,9 @@ def use_react_native! (options={})
 
   if hermes_enabled
     pod 'React-hermes', :path => "#{prefix}/ReactCommon/hermes"
-    if ENV['BUILD_HERMES_SOURCE'] == '1'
-      Pod::UI.puts "[Hermes] Building Hermes from source"
-      hermes_source_path = downloadAndConfigureHermesSource(prefix)
-      pod 'hermes-engine', :path => "#{hermes_source_path}/hermes-engine.podspec"
-    else
-      Pod::UI.warn "[Hermes] Installing Hermes from CocoaPods. The `hermes-engine` pod has been deprecated and will not see future updates."
-      pod 'hermes-engine', '~> 0.11.0'
-    end
+    Pod::UI.puts "[Hermes] Building Hermes from source"
+    hermes_source_path = downloadAndConfigureHermesSource(prefix)
+    pod 'hermes-engine', :path => "#{hermes_source_path}/hermes-engine.podspec"
     pod 'libevent', '~> 2.1.12'
   end