Merge pull request #4879 from ejona86/protoc-artifacts-tls12

protoc-artifacts: Update centos base from 6.6 to 6.9

protoc-artifacts/Dockerfile

@@ -1,4 +1,4 @@
-FROM centos:6.6
+FROM centos:6.9
 
 RUN yum install -y git \
                    tar \