Upgrade to Protobuf 3.5.1 and Protoc 3.5.1-1. (#3921)

protoc-3.5.0-linux-x86_64 introduced GLIBC_2.14 dependency and broke
gRPC release process (https://github.com/google/protobuf/issues/4138).
3.5.1-1 is the proper re-build.
diff --git a/compiler/Dockerfile b/compiler/Dockerfile
index 370006f..d76cb78 100644
--- a/compiler/Dockerfile
+++ b/compiler/Dockerfile
@@ -2,7 +2,7 @@
 
 RUN scl enable devtoolset-1.1 'bash -c "cd /protobuf && \
     git fetch && \
-    git checkout v3.5.0 && \
+    git checkout v3.5.1 && \
     ./autogen.sh && \
     CXXFLAGS=-m32 ./configure --disable-shared --prefix=/protobuf-32 && \
     make clean && make -j$(nproc) && make -j$(nproc) install"'