test: use phantom for dev

Author: yyx990803

Diff for: test/unit/karma.dev.config.js

@@ -2,10 +2,10 @@ var base = require('./karma.base.config.js')
 
 module.exports = function (config) {
   config.set(Object.assign(base, {
-    browsers: ['ChromeHeadless'],
+    browsers: ['PhantomJS'],
     reporters: ['progress'],
     plugins: base.plugins.concat([
-      'karma-chrome-launcher'
+      'karma-phantomjs-launcher'
     ])
   }))
 }