Upgrade vendored assets to CodeMirror 4.6
Bump gem version for release
Upgrade vendored assets to CodeMirror 4.7.
Bump gem version for release
Upgrade vendored assets to CodeMirror 4.8
Bump gem version for release
Update README.md

Use css sprockets in css examples
Fix README code block syntax
Update README with addon example

"Util" has been "addon" since CodeMirror v3.01.
Added Rake task that automates vendoring a new CodeMirror version.