Remove short array syntax and make it compatible for 5.3

Have removed the type callable from 

```php
-    public function __construct(callable $factory)
+    public function __construct($factory)
     {
+        if (! is_callable($factory)) {
+            throw new \Exception("Should be callable");
+        }
         $this->factory = $factory;
     } 
```

https://github.com/harikt/Aura.Input/commit/8f374c1acb243932b089fafef811ddee710fd572

The exception can be moved to its own place.

https://travis-ci.org/harikt/Aura.Input/jobs/8448918

Let me know your comments