Switch to proto3

[varpa89]

#79 (comment)
Mesos 1.4.0 uses proto3
https://issues.apache.org/jira/browse/MESOS-7228

I think it's time to change dependencies =)
#79 (comment)

[ssalinas]

I've created a fork at https://github.com/HubSpot/mesos-rxjava which is upgraded to proto3 if you want to use it @varpa89 , will PR back here as well