Validate pull requests with Travis

Hello, I wrote a tool that can validate README links (valid URLs, not duplicate). It can be run when someone submits a pull request.

It is currently being used by
- https://github.com/vsouza/awesome-ios
- https://github.com/matteocrippa/awesome-swift
- https://github.com/dkhamsing/open-source-ios-apps

Examples
- https://travis-ci.org/matteocrippa/awesome-swift/builds/96526196 ok ✅
- https://travis-ci.org/matteocrippa/awesome-swift/builds/96722421 link redirected / rename 🔴
- https://travis-ci.org/dkhamsing/open-source-ios-apps/builds/96763135 bad link / project deleted 🔴
- https://travis-ci.org/dkhamsing/open-source-ios-apps/builds/95754715 dupe 🔴

If you are interested, connect this repo to https://travis-ci.org/ and add a [`.travis.yml`](https://github.com/dkhamsing/open-source-ios-apps/blob/master/.travis.yml) file to the project.

See https://github.com/dkhamsing/awesome_bot for options, more information
Feel free to leave a comment :smile:

Review the referenced .travis.yml example and the awesome_bot options first. Configure this repository to run README link validation through Travis on pull requests; done means the check detects invalid, redirected, deleted, or duplicate links.

Written by the indexing model from the issue text.