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/build.gradle b/build.gradle
index 40ef909..ac71a01 100644
--- a/build.gradle
+++ b/build.gradle
@@ -98,8 +98,8 @@
 
         nettyVersion = '4.1.17.Final'
         guavaVersion = '19.0'
-        protobufVersion = '3.5.0'
-        protocVersion = protobufVersion
+        protobufVersion = '3.5.1'
+        protocVersion = '3.5.1-1'
         protobufNanoVersion = '3.0.0-alpha-5'
 
         configureProtoCompilation = {