Bump protobuf dependency to 3.0.0-beta-3

This allows us to play with zero-copy and proto3 support for lite.
Unfortunately, it introduced some warnings, so deprecated warnings are
now ignored for benchmarks and interop-testing.
diff --git a/COMPILING.md b/COMPILING.md
index 9fd2257..f9cecca 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -27,13 +27,13 @@
 generation. Most users only need to use `skipCodegen=true` as discussed above.
 
 ### Build Protobuf
-The codegen plugin is C++ code and requires protobuf 3.0.0-beta-2.
+The codegen plugin is C++ code and requires protobuf 3.0.0-beta-3.
 
 For Linux, Mac and MinGW:
 ```
 $ git clone https://github.com/google/protobuf.git
 $ cd protobuf
-$ git checkout v3.0.0-beta-2
+$ git checkout v3.0.0-beta-3
 $ ./autogen.sh
 $ ./configure
 $ make
@@ -72,16 +72,16 @@
 Gradle to find protobuf:
 ```
 .\gradlew install ^
-    -PvcProtobufInclude=C:\path\to\protobuf-3.0.0-beta-2\src ^
-    -PvcProtobufLibs=C:\path\to\protobuf-3.0.0-beta-2\vsprojects\Release ^
+    -PvcProtobufInclude=C:\path\to\protobuf-3.0.0-beta-3\src ^
+    -PvcProtobufLibs=C:\path\to\protobuf-3.0.0-beta-3\vsprojects\Release ^
     -PtargetArch=x86_32
 ```
 
 Since specifying those properties every build is bothersome, you can instead
 create ``<project-root>\gradle.properties`` with contents like:
 ```
-vcProtobufInclude=C:\\path\\to\\protobuf-3.0.0-beta-2\\src
-vcProtobufLibs=C:\\path\\to\\protobuf-3.0.0-beta-2\\vsprojects\\Release
+vcProtobufInclude=C:\\path\\to\\protobuf-3.0.0-beta-3\\src
+vcProtobufLibs=C:\\path\\to\\protobuf-3.0.0-beta-3\\vsprojects\\Release
 targetArch=x86_32
 ```