Fix Slack badge image insecure content warning

Image is no longer insecure when viewing REAMDE off of github (which does not serve the original image URLs).

README.md

@@ -4,7 +4,7 @@
 [![Coverage Status](https://coveralls.io/repos/istanbuljs/nyc/badge.svg?branch=)](https://coveralls.io/r/istanbuljs/nyc?branch=master)
 [![NPM version](https://img.shields.io/npm/v/nyc.svg)](https://www.npmjs.com/package/nyc)
 [![Conventional Commits](https://img.shields.io/badge/Conventional%20Commits-1.0.0-yellow.svg)](https://conventionalcommits.org)
-[![community slack](http://devtoolscommunity.herokuapp.com/badge.svg)](http://devtoolscommunity.herokuapp.com)
+[![community slack](https://devtoolscommunity.herokuapp.com/badge.svg)](http://devtoolscommunity.herokuapp.com)
 
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