extras/tools: Bump the required protoc version

Contribs build protobuf 3.1.0 and assumes that the host environment
already has got a new enough protoc version.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

extras/tools/bootstrap

@@ -80,7 +80,7 @@ check yasm
 check_tar
 check ragel
 check_sed
-check protoc 2.6.0
+check protoc 3.1.0
 check ant
 check xz
 check bison 3.0.0