Prep vision-0.25.0 release. (#3537)
Prep vision-0.25.0 release. (#3537)

Note that the version number was bumped prematurely in PR 72e6e522.
calpeyser/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,jonparrott/gcloud-python,tseaver/gcloud-python,dhermes/gcloud-python,tartavull/google-cloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,calpeyser/google-cloud-python,GoogleCloudPlatform/gcloud-python,tseaver/gcloud-python,tseaver/google-cloud-python,GoogleCloudPlatform/gcloud-python,tartavull/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,jonparrott/gcloud-python