Increase maxBuffer for pngcrusher to be able to run Font Awesome

I was attempting to run [Font Awesome](https://github.com/FortAwesome/Font-Awesome) through Grunticon and was running into the following:

> Error: stderr maxBuffer exceeded.

After looking through some Node.js documentation I stumbled upon [maxBuffer](http://nodejs.org/api/child_process.html) and found that increasing it for the pngcrusher child process fixed the issue.

The default is 200 and I found that 250 seemed to be the smallest increase that works with running Font Awesome. I tried to extract it as much as possible so anyone else can easily update themselves.