Add mcgreevy to CONTRIBUTORS.
Add mcgreevy to CONTRIBUTORS.

Change-Id: I2b03b75717ff058db616849b87f9e11ffdd2bd93

roth1002/gcloud-golang,googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,googleapis/google-cloud-go,roth1002/gcloud-golang,ptomasroos/gcloud-golang,hsinhoyeh/gcloud-golang,googleapis/google-cloud-go,blowmage/gcloud-golang,googleapis/google-cloud-go,Maksadbek/gcloud-golang,blowmage/gcloud-golang,michael-grosshans/google-cloud-go,Maksadbek/gcloud-golang,ptomasroos/gcloud-golang,michael-grosshans/google-cloud-go