switch to broccoli-persistent-filter

https://github.com/stefanpenner/broccoli-persistent-filter

fix GH-8
close GH-9

index.js

@@ -3,8 +3,9 @@
 var path = require('path');
 
 var CleanCssPromise = require('clean-css-promise');
-var Filter = require('broccoli-filter');
+var Filter = require('broccoli-persistent-filter');
 var inlineSourceMapComment = require('inline-source-map-comment');
+var jsonStableStringify = require('json-stable-stringify');
 
 function CleanCSSFilter(inputTree, options) {
   if (!(this instanceof CleanCSSFilter)) {
@@ -25,6 +26,23 @@ CleanCSSFilter.prototype.constructor = CleanCSSFilter;
 CleanCSSFilter.prototype.extensions = ['css'];
 CleanCSSFilter.prototype.targetExtension = 'css';
 
+CleanCSSFilter.prototype.baseDir = function() {
+  return __dirname;
+};
+
+CleanCSSFilter.prototype.optionsHash = function() {
+  if (!this._optionsHash) {
+    this._optionsHash = jsonStableStringify(this.options);
+  }
+
+  return this._optionsHash;
+};
+
+CleanCSSFilter.prototype.cacheKeyProcessString = function(string, relativePath) {
+  return this.optionsHash() +
+         Filter.prototype.cacheKeyProcessString.call(this, string, relativePath);
+};
+
 CleanCSSFilter.prototype.build = function() {
   var srcDir = this.inputPaths[0];
   var relativeTo = this.options.relativeTo;

package.json

@@ -30,9 +30,10 @@
     "whitespace"
   ],
   "dependencies": {
-    "broccoli-filter": "^1.2.0",
+    "broccoli-persistent-filter": "^1.1.6",
     "clean-css-promise": "^1.0.0",
-    "inline-source-map-comment": "^1.0.5"
+    "inline-source-map-comment": "^1.0.5",
+    "json-stable-stringify": "^1.0.0"
   },
   "devDependencies": {
     "@shinnn/eslintrc-node": "^1.0.2",