Travis should only build API or Dashboard image when respective code has changed

[ckadner]

From @Tomcli:

... we can leverage the script we have wrote for kfp-tekton. This way we don't need to build the image if the api/ui code hasn't changed

https://github.com/kubeflow/kfp-tekton/blob/master/.travis.yml#L73-L95

Originally posted by @Tomcli in #27 (comment)

[ckadner]

This is a good idea @Tomcli -- let me know if you want to add this? Otherwise I will create a PR for it.

[Tomcli]

I can put up something simple to start with. What we have in kfp-tekton is to check whether any source file in our dockerfiles has changed, if so run the docker build. You can update the script later on if you want to check for diff in a specific dockerfile.