Added phantomjssmith to solve `Fatal error: Unsupported interlace method`

Was getting `Fatal error: Unsupported interlace method` whenever I would run `grunt sprite`.

I was able to solve this by first running `npm install -g phantomjs`, then `npm install phantomjssmith --save`.

Devs who are compiling the sprites. Does that make sense to you? Should I add a line to the README about it?