Fix plugins list with up to date entries

Replaces http://isaacs.iriscouch.com to https://skimdb.npmjs.com in order to get up to date gruntplugins from the npm registry.

Ref http://blog.npmjs.org/post/75707294465/new-npm-registry-architecture (isaacs.iriscouch.com is only a downstream mirror with not frequently updates)
Ref #92
Fixes #131