Tweak AppVeyor testing

* changed `autocrlf` to `true`
* ~~changed the badges to SVG~~ submitted a patch upstream for this, see https://github.com/gruntjs/grunt-contrib-internal/pull/16
* enable caching

Note that this has reduced the build time from around 4 min 20 sec to 1 min 25 sec.

/CC @gruntjs/contrib-less 

P.S. I would backport this for the other contrib plugins, but I don't have access there.