Remove invalid links (#979)

Author: mccricardo

README.md

@@ -77,7 +77,6 @@ Companies Using Slate
 * [Travis-CI](https://docs.travis-ci.com/api/)
 * [Greenhouse](https://developers.greenhouse.io/harvest.html)
 * [Woocommerce](http://woocommerce.github.io/woocommerce-rest-api-docs/)
-* [Appium](http://appium.io/slate/en/master)
 * [Dwolla](https://docs.dwolla.com/)
 * [Clearbit](https://clearbit.com/docs)
 * [Coinbase](https://developers.coinbase.com/api)