Add OpenAPI and Swagger validation to Travis CI

[blgm]

What is the problem this PR solves? Fixes issue #664 by adding validation of Swagger and OpenAPI in TravisCI

This is done using a validator from DockerHub, and it does therefore add an external dependency to CI.

Note that due to errors in the Swagger and OpenAPI validation covered by #662, the Travis CI build fails with this change. Here is an example: https://travis-ci.com/blgm/servicebroker/builds/111944021

Checklist:

• [ ] The swagger.yaml doc has been updated with any required changes Not applicable
• [ ] The openapi.yaml doc has been updated with any required changes Not applicable

[cfdreddbot]

:white_check_mark: Hey blgm! The commit authors and yourself have already signed the CLA.

[blgm]

After cherry-picking the fix from #667, Travis is now green!