update npm badge

[kentcdodds]

Using shields gives an svg which looks much better (especially on retina screens):

before:

after:

[coveralls]

Coverage remained the same at 91.714% when pulling 87bf6c3 on kentcdodds:patch-1 into 4dd253a on JedWatson:master.

[JedWatson]

👍 thanks @kentcdodds!