protobuf 2.5.0 started warning that we would need to enable the
'allow_alias' option on this enum due to the duplicate enum values, and
protobuf 2.6.0 turned this into an error. Turn this option on, now that
protobuf 2.5.0 is more common (e.g., it's now in Ubuntu 14.04 LTS). This
will break with older protobuf versions, so we now require >= 2.5.0.

We should now see diagnostics like these disappear from the build log:

    [libprotobuf ERROR google/protobuf/descriptor.cc:4153] "foo.VALUE_B" uses the same enum value as "foo.VALUE_A". If this is intended, set 'option allow_alias = true;' to the enum definition.

Based on a patch from Ilya Lipnitskiy.