http -> https for external links

README.md

 [Grafana](http://grafana.org) [![Build Status](https://api.travis-ci.org/grafana/grafana.svg)](https://travis-ci.org/grafana/grafana) [![Coverage Status](https://coveralls.io/repos/grafana/grafana/badge.png)](https://coveralls.io/r/grafana/grafana) [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/grafana/grafana?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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 Grafana is An open source, feature rich metrics dashboard and graph editor for