Fix unencoded ✭ character in img src

[papandreou]

Fix unencoded star character in img src url (https://img.shields.io/github/stars/webpack/memory-fs.svg?label=✭&style=flat-square&maxAge=3600)

Caused npm run linkcheck to break.

This is a hopefully nicer alternative to disabling the link check: #2759

We'll look into making hyperlink either tolerate this kind of invalid url (if browsers tolerate it), or throw a specific error message instead of an internal error.

[Munter]

When the tests could run without throwing, another problem showed itself: webpack-contrib/cache-loader#60

I'll merge this one though, because it clearly fixes a problem, and the broken link that was introduced is a part of an external repository