Use CDN with proper Content-Type headers for epeli underscore library

See http://rawgit.com/faq -- latest version of Chrome is currently giving this error in telemetry graphs:  "Refused to execute script from 'http://raw.github.com/epeli/underscore.string/2.3.3/dist/underscore.string.min.js' because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled."