Update autoprefixer map option

Accommodate the grunt-autoprefixer 0.8.0 options refactor
by passing an object (instead of string) to map option
a la nDmitry/grunt-autoprefixer@c658fb7
and nDmitry/grunt-autoprefixer@1b7af91
Merge pull request #1082 from fullyint/update-autoprefixer

Update autoprefixer map option