Update Podspec to 0.9.1 (#496)

Planning to cut a new minor bump soon.

SwiftGRPC.podspec

@@ -11,7 +11,7 @@
 
 Pod::Spec.new do |s|
   s.name = 'SwiftGRPC'
-  s.version = '0.9.0'
+  s.version = '0.9.1'
   s.license     = { :type => 'Apache License, Version 2.0',
                     :text => <<-LICENSE
                       Copyright 2018, gRPC Authors. All rights reserved.