Merge pull request #2302 from jcanizales/point-to-protobuf-head

Point tests to protobuf head

src/objective-c/tests/Podfile

@@ -1,6 +1,7 @@
 source 'https://github.com/CocoaPods/Specs.git'
 platform :ios, '8.0'
 
+pod 'Protobuf', :path => "../../../third_party/protobuf"
 pod 'gRPC', :path => "../../.."
 pod 'RemoteTest', :path => "../generated_libraries/RemoteTestClient"
 pod 'RouteGuide', :path => "../generated_libraries/RouteGuideClient"