Consistent badge style on README.md

[amio]

Prerequisites

• Put an X between the brackets on this line if you have done all of the following:
  • Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
  • Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
  • Checked the FAQs on the message board for common solutions: https://discuss.atom.io/c/faq
  • Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=is%3Aissue+user%3Aatom
  • Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages

Description

Currently there's 3 badges in README.md shows building status, but the style looks mixed:

What about using a more consistent style badge? like:

[![Linux build][linux-build-src]][linux-build-href]
[![Windows build][windows-build-src]][windows-build-href]
[![macOS build][macos-build-src]][macos-build-href]

[linux-build-src]: https://badgen.net/travis/atom/watcher/master?label=Linux
[linux-build-href]: https://travis-ci.org/atom/watcher
[windows-build-src]: https://badgen.net/appveyor/ci/atom/watcher/master?label=Windows
[windows-build-href]: https://ci.appveyor.com/project/Atom/watcher/branch/master
[macos-build-src]: https://badgen.net/circleci/github/atom/watcher/master?label=macOS
[macos-build-href]: https://ci.appveyor.com/project/Atom/watcher/branch/master

And maybe the greenkeeper badge too (Greenkeeper's vs Badgen's):

[![Greenkeeper enabled][greenkeeper-src]][greenkeeper-href]

[greenkeeper-src]: https://badgen.net/badge/Greenkeeper/enabled/green
[greenkeeper-href]: https://greenkeeper.io/

Additional Information

shields.io's badges often broken recently, badgen.net is a new service that aims to be a fast and reliable alternative.

Disclaimer: Badgen's author here :D