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:

Start by reviewing the linked .travis.yml example and the awesome_bot options, then determine the Travis configuration needed for this repository. Done means pull requests run README link validation and flag invalid, redirected, deleted, or duplicate links.

Written by the indexing model from the issue text.