Use Circle CI for faster builds. (#181)

Move to Circle CI from Travis CI.

.travis/deploy_dockerhub.sh

@@ -7,12 +7,12 @@ deploy () {
 }
 
 
-if [ "$TRAVIS_EVENT_TYPE" = "cron" ]; then
+if [ "$EVENT_TYPE" = "cron" ]; then
 		deploy "rolling"
-elif [ "$TRAVIS_BRANCH" = "master" ]; then
+elif [ "$BRANCH" = "master" ]; then
 		deploy "latest"
 else
 		# If the branch isn't master and this was not a cron job, no need to deploy.
-		echo "TRAVIS_EVENT_TYPE = $TRAVIS_EVENT_TYPE, TRAVIS_BRANCH = $TRAVIS_BRANCH"
+		echo "EVENT_TYPE = $EVENT_TYPE, BRANCH = $BRANCH"
 		exit 0
 fi