Include CI for master branch and PR's

[ariesshrimp]

In accordance with #19 (comment),
includes config for TravisCI to run test reports on the master branch
and PR's to the master branch.

• Includes .travis.yml config file
• Includes current CI status badge in README

Closes #19.

Note: Travis was not building the yarn cache successfully until I rebuilt the lock file. npm install worked fine though.

The currently failing tests are fixed in https://github.com/facebook/prop-types/pull/129/files

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need the corporate CLA signed.

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[ariesshrimp]

Someone will of course need to go and enable the repo on travis-ci.org on facebook's behalf, which I don't have access to.

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[gaearon]

Thank you!