fixed css relative path to minicolors sprite

Hi Guys,

I'm working with @chrisfilo on [NeuroVault](https://github.com/NeuroVault/NeuroVault), and I've been doing some updates on our utilization of pycortex.

While upgrading our branch, I noticed this broken css path to the color picker sprite.  The file name seems to changed, which broke the relative paths used in webgl.make_static().  I think it only affects the statically generated views.

Thanks for the great tool. :+1: 

Cheers,
Gabriel