Support Ruby 2.2.0

ref: https://bugs.ruby-lang.org/issues/10314 #1902

Author: tonytonyjan

core/lib/compass/core/caniuse.rb

@@ -69,7 +69,7 @@ def all_prefixes(browser)
   end
 
   # returns the prefixes needed by the list of browsers given
-  def prefixes(browsers = browsers)
+  def prefixes(browsers = browsers())
     result = browsers.map{|b| all_prefixes(b) }
     result.flatten!
     result.uniq!