Fix autoprefixer invocation

Before this patch autoprefixer processed files with default `browserSupport` because of incorrect `apply` with object argument instead of array.
And so we stepped in this code branch
https://github.com/postcss/autoprefixer-core/blob/master/lib/autoprefixer.coffee#L24
instead of this
https://github.com/postcss/autoprefixer-core/blob/master/lib/autoprefixer.coffee#L21

And instead of wrapping arguments in array, invocation logic borrowed from grunt-autoprefixer which seems to be closer to [official usage recommendation](https://github.com/postcss/autoprefixer-core#usage).

Here's results after processing same css file with option browserSupport: **"last 8 Chrome versions"**
(before/after)
https://yadi.sk/i/x4Fz01G_dgviQ
https://yadi.sk/i/LhLlmbXOdgvi4
Notice that `-moz-box-sizing`, `-webkit-linear-gradient`, `-ms/moz-user-select` props removed and `-moz-font-kerning` no longer being added as expected.