Make iOS 9 compatible again. (#874)

Summary:
React Native v0.62.0’s template still uses iOS 9 as deployment target, bumping it to 10 this late in the release cycle is probably not a good idea, so instead I made this tiny change to make it iOS 9 compatible again.

⚠️  I made this change on top of the `v0.32.2` **tag**, as `master` was giving me a build failure and in any case it would be better to release _just_ this change as a patch release so the RN `v0.62.0-rc.3` release can go out without having to test Flipper as thoroughly again.

## Changelog

Make FlipperKit compatible with iOS 9 again.
Pull Request resolved: https://github.com/facebook/flipper/pull/874

Test Plan:
- `pod install` with a Podfile that has iOS 9 as its deployment target passes again.
- Building the Flipper Sample works.
- Building with a RN application created with the `v0.62.0-rc.3` template works.

Reviewed By: priteshrnandgaonkar

Differential Revision: D20307073

Pulled By: passy

fbshipit-source-id: 0dd02dfb810b7382c52ca81b8f1322f807fb3f29

FlipperKit.podspec

@@ -17,7 +17,7 @@ Pod::Spec.new do |spec|
   spec.source = { :git => 'https://github.com/facebook/Sonar.git',
                   :tag=> "v"+flipperkit_version }
   spec.module_name = 'FlipperKit'
-  spec.platforms = { :ios => "10.0" }
+  spec.platforms = { :ios => "9.0" }
   spec.default_subspecs = "Core"
 
   # This subspec is necessary since FBDefines.h is imported as <FBDefines/FBDefines.h>