fix a typo in `engines.node` inside `package.json`

The “greater or equal” sign should be written as `>=`, not as `=>`.

This pull request gets rid of a warning (`npm WARN engine colors.css@2.0.2: wanted: {"node":"=>0.10.22"} (current: {"node":"0.10.30","npm":"1.4.27"})`) issued by npm in console.