Add a huge GitHub css to ensure myth is fast (at least for parse/stringify)

I assume that for a big CSS like that, 300ms is a max on a developer machine.
What do you think about that @ianstormtaylor ?

The build is failing because travis is a bit slow a it take twice the time it take on my setup (iMac 3.2ghz icore5).
More than 400ms.

What value will you consider has fast for a huge CSS like this one ?
Maybe we should not add this a test ?