doc(readme): use SVG badge instead of PNG badge

SVG badges look beautiful on retina displays.
chore(package): remove unnecessary main field
feat(package): update google-cdn and grunt plugins
feat(task): use each-async instead of grunt.util

grunt.util.async is deprecated.
fix(grunt): fix target file array of jshint task
chore(jshint): update .jshintrc

https://github.com/jshint/jshint/releases/tag/2.5.0