Add travis ci configuration.

[nanliu]

This is an example travis config to address the release issues per #60:

• create a new docker tag called master for testing development releases. (we can change this tag name to dev or something else)
• for every new git tag create a matching docker tag and update the latest tag.

Here's the example git tag and the corresponding docker hub image. There are two steps to enable this:

1. enable travis ci for this repo.
2. use travis cli and configure the docker hub login:

   $ travis env set -r 'PalmStoneGames/kube-cert-manager' DOCKER_USERNAME <username>
   $ travis env set -r 'PalmStoneGames/kube-cert-manager' DOCKER_PASSWORD <password>

   To verify:

   travis env list -r 'PalmStoneGames/kube-cert-manager'
   # environment variables for PalmStoneGames/kube-cert-manager
   DOCKER_USERNAME=[secure]
   DOCKER_PASSWORD=[secure]

[ahmetb]

I think the current codebuild+buildspec+travis, which all of them keep duplicating the build steps, is not ideal.

How about creating a makefile with targets like get-deps, build, docker-image then each builder can use outputs of those things to tag/push images?