Fix Core version requirement. (#1871)

These pods depend on FirebaseCore's interop APIs that were introduced
in FirebaseCore version 5.1. This wasn't picked up before since
CocoaPods automatically matched the largest minor version available but
the podspec should accurately represent the version requirements.

FirebaseAuth.podspec

@@ -62,7 +62,7 @@ supports email and password accounts, as well as several 3rd party authenticatio
   s.framework = 'SafariServices'
   s.framework = 'Security'
   s.dependency 'FirebaseAuthInterop', '~> 1.0'
-  s.dependency 'FirebaseCore', '~> 5.0'
+  s.dependency 'FirebaseCore', '~> 5.1'
   s.dependency 'GoogleUtilities/Environment', '~> 5.2'
   s.dependency 'GTMSessionFetcher/Core', '~> 1.1'
 end