Cleanup .travis.yml

--buildpack help improvements

* Inform users that they can pass a directory
* Inform users that they can pass multiple --buildpack flags

Signed-off-by: Emily Casey <ecasey@pivotal.io>

Update README to reflect latest UX

citravis-test-and-windows

Adds create-builder command

* switches default builder image to packs/samples
* change --build-image flag to --builder

[buildpacks#23]
[buildpacks#24]

Pulls base image before exporting