Correct `npm cache clean` command

Change `npm cache clear` -> `npm cache clean` command in "Motivation / Reduce diff churn" section. 

I'm assuming this is a typo as `npm cache clean` is listed as a command in the [npm-cache](https://www.npmjs.org/doc/cli/npm-cache.html) docs, but not `npm cache clear`.
Merge pull request #19 from robatron/patch-1

Correct `npm cache clean` command in the README