Prevent lazyloading (#67)

If not set, the relationship is not lazyloaded and you can get errors like 
```
Attempted to lazy load [versionable] on model [Overtrue\LaravelVersionable\Version] but lazy loading is disabled.```
if enforced.

src/Version.php

@@ -36,6 +36,10 @@ class Version extends Model
         'contents' => 'json',
     ];
 
+    protected $with = [
+        'versionable',
+    ];
+
     protected static function booted()
     {
         static::creating(function (Version $version) {