Add badges for npm dependencies

More badges sucks a bit, but Gemnasium is now only watching this
project's RubyGems dependencies - which are less important.

This can be fixed with the Gemnasium API, but it's easier to not
mess around with tokens and just use a different service.

http://gemnasium.freshdesk.com/support/solutions/articles/101939-how-to-check-more-than-one-package-type-ie-npm-and-rubygem-for-a-project-