Fix CSS variable support check (take 2)

Author: Thibaut

assets/javascripts/app/app.coffee

@@ -245,7 +245,7 @@
         matchMedia:         !!window.matchMedia
         insertAdjacentHTML: !!document.body.insertAdjacentHTML
         defaultPrevented:     document.createEvent('CustomEvent').defaultPrevented is false
-        cssVariables:         CSS.supports and CSS.supports('--t', '0')
+        cssVariables:         CSS.supports and CSS.supports('(--t: 0)')
 
       for key, value of features when not value
         Raven.captureMessage "unsupported/#{key}", level: 'info'