Deployment script should not execute for pull requests

I can't think of a reason why it would be desirable; and I didn't realize it was until just now. Probably a check is needed right before the post is made to deployasaur.us:

deployasaur.us/public/deploy.sh

Line 18 in a625d5d

print "howdy $repository build $build! nice to hear from you."

I think there's some travis environmental variable we can check and exit with some message.