Update has-many.js (#598)

There was an issue with checking a hasMany for me, I'm not 100% sure on the exact circumstances but it was triggered after an upgrade to EmberJS 3.1+. Importing get, rather than assuming the object has the get method, fixed things nicely and is arguably a smart way to proceed anyway.

addon/validators/has-many.js

@@ -1,5 +1,6 @@
 import Base from 'ember-cp-validations/validators/base';
 import { isPromise } from 'ember-cp-validations/utils/utils';
+import { get } from '@ember/object';
 
 /**
  *  <i class="fa fa-hand-o-right" aria-hidden="true"></i> [See All Options](#method_validate)
@@ -55,7 +56,7 @@ const HasMany = Base.extend({
         return value.then(models => this.validate(models, ...args));
       }
 
-      return value.map(m => m.get('validations'));
+      return value.map(m => get(m, 'validations'));
     }
 
     return true;