deliberate ES6 syntax

(rationale: don't let people install @2 in a build system that will not alert them that we have moved to ES6, only to cause trouble with a later release.)

src/scheme.js

@@ -1,11 +1,5 @@
 export default function(ranges) {
-  ranges = ranges.map(function(colors) {
-    return colors.match(/.{6}/g).map(function(x) {
-      return "#" + x;
-    });
-  });
-  var n0 = ranges[0].length;
-  return function(n) {
-    return ranges[n - n0];
-  };
+  ranges = ranges.map(colors => colors.match(/.{6}/g).map(x => `#${x}`));
+  const n0 = ranges[0].length;
+  return n => ranges[n - n0];
 }