Merge pull request #545 from thoughtbot/js-readme-update

Add 10.13 to list of supported operating systems in the readme.

README.md

@@ -16,6 +16,7 @@ We support:
 * macOS Yosemite (10.10)
 * macOS El Capitan (10.11)
 * macOS Sierra (10.12)
+* macOS High Sierra (10.13)
 
 Older versions may work but aren't regularly tested.
 Bug reports for older versions are welcome.