Change build script to build latest release versions if not in CI (#115)

Build.sh can build the latest versions of the images using the VERSION file under the two repos (core & web).

.travis/deploy_dockerhub.sh

@@ -1,4 +1,10 @@
 #!/bin/sh
-docker login -e $DOCKER_EMAIL -u $DOCKER_USER -p $DOCKER_PASS
-docker push maxking/mailman-web
-docker push maxking/mailman-core
+
+if [[ "$TRAVIS_BRANCH" == "master" ]]
+then
+	docker login -e $DOCKER_EMAIL -u $DOCKER_USER -p $DOCKER_PASS
+	docker push maxking/mailman-web
+	docker push maxking/mailman-core
+else
+	echo "Non-master branch aren't pushed to DockerHub..."
+fi