[infra] automatically publish new images

This tags new images every commit (via Travis) as latest, and every
tag gets pushed as a separate tag as well.

.travis-deploy.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+docker login -u "$DOCKER_USERNAME" -p "$DOCKER_PASSWORD"
+docker push directxman12/k8s-prometheus-provider:latest
+if [[ -n $TRAVIS_TAG ]]; then
+    docker push directxman12/k8s-prometheus-provider:$TRAVIS_TAG
+fi

.travis.yml

@@ -21,3 +21,9 @@ script: make verify
 cache:
   directories:
   - ~/.glide
+
+deploy:
+  provider: script
+  script: .travis-deploy.sh
+  on:
+    branch: master