[bugfix release] fix using helpers without default Inflector

The singularize and pluralize helpers previously imported the
helpers previously without a default Ember Inflector instance,
which would cause issues for Ember CLI users.

refs emberjsGH-74

Author: Stanley Stuart

addon/lib/helpers/pluralize.js

@@ -1,4 +1,4 @@
-import { pluralize } from '../system/string';
+import { pluralize } from 'ember-inflector';
 import makeHelper from '../utils/make-helper';
 
 /**

addon/lib/helpers/singularize.js

@@ -1,4 +1,4 @@
-import { singularize } from '../system/string';
+import { singularize } from 'ember-inflector';
 import makeHelper from '../utils/make-helper';
 
 /**