Add jepso-ci

This P.R. will add [jepso-ci](https://jepso-ci.com) to your repository.  It lets you put a badge like:

[![build status](https://jepso-ci.com/jepso-ci-examples/es6-shim.svg)](https://jepso-ci.com/jepso-ci-examples/es6-shim)

In the readme of your repository.

To enable jepso-ci just accept this pull request and then head over to [settings/hooks](https://github.com/paulmillr/es6-shim/settings/hooks) select **WebHookURLs** and enter `https://jepso-ci.com/api/hook` then click **Update Settings**.  To have jepso-ci build your repository immediately just click **Test Hook**.  You can also rely on jepso-ci re-building your repository every time you push a commit.

To see live build status of your repository once the build has started just go to https://jepso-ci.com/paulmillr/es6-shim

You can see an example of what the badge would look like in your readme at [jepso-ci-examples/es6-shim](https://github.com/jepso-ci-examples/es6-shim).