Made protobuf version 3+ a hard requirement for building Mesos.
Made protobuf version 3+ a hard requirement for building Mesos.

Review: https://reviews.apache.org/r/64822/

chhsia0/mesos,dforsyth/mesos,gsantovena/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,asamerh4/mesos,gsantovena/mesos,jpeach/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,asamerh4/mesos,Gilbert88/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,kaysoky/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,asamerh4/mesos,jpeach/mesos,asamerh4/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,asamerh4/mesos,reneploetz/mesos,jpeach/mesos,jpeach/mesos,chhsia0/mesos,abudnik/mesos,andschwa/mesos,asamerh4/mesos,kaysoky/mesos,gsantovena/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,abudnik/mesos,chhsia0/mesos,asamerh4/mesos,Gilbert88/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos