Bump protobuf to 3.1.0

Also removed warnings about protoc version matching runtime, since this
is no longer supposed to be a problem (starting with 3.0.0-beta-4) and
all our tests ran fine when using protoc 3.0.2 with protobuf runtime
3.1.0.

Fixes #2316
diff --git a/compiler/Dockerfile b/compiler/Dockerfile
index 1aa4099..b04c0c3 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 v3.0.2 && \
+    git checkout v3.10 && \
     ./autogen.sh && \
     CXXFLAGS=-m32 ./configure --disable-shared --prefix=/protobuf-32 && \
     make clean && make -j$(nproc) && make -j$(nproc) install"'