Fix MySQL Schema Grammar $modifiers order

Charset/Collate must be placed before VirtualAs/StoredAs.

e.g.

```php
$table->string('hash', 64)->charset('ascii')->storedAs('sha2(`url`,256)')
```

According to Sequel Pro, this is equivalent to:

```sql
`hash` varchar(64) CHARACTER SET ascii GENERATED ALWAYS AS (sha2(`url`,256)) STORED
```

src/Illuminate/Database/Schema/Grammars/MySqlGrammar.php

@@ -15,7 +15,7 @@ class MySqlGrammar extends Grammar
      * @var array
      */
     protected $modifiers = [
-        'Unsigned', 'VirtualAs', 'StoredAs', 'Charset', 'Collate', 'Nullable',
+        'Unsigned', 'Charset', 'Collate', 'VirtualAs', 'StoredAs', 'Nullable',
         'Default', 'Increment', 'Comment', 'After', 'First', 'Srid',
     ];