use valid sans-serif value

[kdoh]

Because $font_sans_serif was defined as a string in dc.scss the browser would look for a font called "sans-serif" instead of using a system sans-serif typeface. We recently discovered this after bumping our dc.js version and noticed our axis labels had changed in metabase/metabase#4115

[gordonwoodhull]

Thanks @kdoh, good catch, that Times definitely doesn't look so sharp at that tiny size!

I guess we missed this when refactoring to Sass in #1049.

Released in 2.0.1.