Unify badge style
The "PRs Welcome" badge in README.md was flat-styled.  Change to the default style of all other badges in the README file
Mycroft [![Build Status](https://travis-ci.org/MycroftAI/mycroft-core.svg?branch=master)](https://travis-ci.org/MycroftAI/mycroft-core) [![Coverage Status](https://coveralls.io/repos/github/MycroftAI/mycroft-core/badge.svg?branch=dev)](https://coveralls.io/github/MycroftAI/mycroft-core?branch=dev)
-[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com)
+[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](http://makeapullrequest.com)
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Mycroft is a hackable open source voice assistant.
