Update protobuf submodule to 3.0.0-beta-4

Which has the fix for oneofs when using dynamic frameworks in ObjectiveC

.gitmodules

@@ -4,7 +4,7 @@
 [submodule "third_party/protobuf"]
 	path = third_party/protobuf
 	url = https://github.com/google/protobuf.git
-	branch = 3.0.0-beta-3
+	branch = 3.0.0-beta-4
 [submodule "third_party/gflags"]
 	path = third_party/gflags
 	url = https://github.com/gflags/gflags.git

examples/cpp/helloworld/Makefile

@@ -97,7 +97,7 @@ ifneq ($(HAS_VALID_PROTOC),true)
 	@echo "Please install Google protocol buffers 3.0.0 and its compiler."
 	@echo "You can find it here:"
 	@echo
-	@echo "   https://github.com/google/protobuf/releases/tag/v3.0.0-beta-3.3"
+	@echo "   https://github.com/google/protobuf/releases/tag/v3.0.0-beta-4"
 	@echo
 	@echo "Here is what I get when trying to evaluate your version of protoc:"
 	@echo

examples/cpp/route_guide/Makefile

@@ -86,7 +86,7 @@ ifneq ($(HAS_VALID_PROTOC),true)
 	@echo "Please install Google protocol buffers 3.0.0 and its compiler."
 	@echo "You can find it here:"
 	@echo
-	@echo "   https://github.com/google/protobuf/releases/tag/v3.0.0-beta-3.3"
+	@echo "   https://github.com/google/protobuf/releases/tag/v3.0.0-beta-4"
 	@echo
 	@echo "Here is what I get when trying to evaluate your version of protoc:"
 	@echo

tools/run_tests/sanity/check_submodules.sh

@@ -45,7 +45,7 @@ cat << EOF | awk '{ print $1 }' | sort > $want_submodules
  05b155ff59114735ec8cd089f669c4c3d8f59029 third_party/gflags (v2.1.0-45-g05b155f)
  c99458533a9b4c743ed51537e25989ea55944908 third_party/googletest (release-1.7.0)
  f8ac463766281625ad710900479130c7fcb4d63b third_party/nanopb (nanopb-0.3.4-29-gf8ac463)
- bdeb215cab2985195325fcd5e70c3fa751f46e0f third_party/protobuf (v3.0.0-beta-3.3)
+ 4b36284bdfdc77896d7c4cb9645433cf86efb069 third_party/protobuf (v3.0.0-beta-4-3-g4b36284)
  50893291621658f355bc5b4d450a8d06a563053d third_party/zlib (v1.2.8)
 EOF