Added a TODO entry for protobuf version numbering.

Change-Id: I414d85d5d4a0b78c0f897beb3949b4237069da08
Reviewed-on: http://review.couchbase.org/38949
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>