Removed pointer receiver from UPID.String() method to fix race conditions detected by `go build -race'.  Resolves https://github.com/mesos/mesos-go/issues/184.
Removed pointer receiver from UPID.String() method to fix race conditions detected by `go build -race'.  Resolves https://github.com/mesos/mesos-go/issues/184.

tnachen/mesos-go,drewkerrigan/mesos-go,chrisjs/mesos-go,jdef/mesos-go,ilackarms/mesos-go,tnachen/go-mesos,mesos/mesos-go,mesosphere/mesos-go,basho-labs/mesos-go