Memory leak when allowQueue=false

If allowQueue parameter set to false Toasty uses lastToast static variable to save previous Toast object which holds reference to Context causing memory leak. Leak is reported by com.squareup.leakcanary library.

You need to set allowQueue to true to eliminate leak (no static reference to Context).

```
// Causing leak
Toasty.custom(_context, _text,
ContextCompat.getDrawable(_context, R.drawable.ic_alert_white_36dp),
ContextCompat.getColor(_context, R.color.warn_background),
ContextCompat.getColor(_context, R.color.defaultTextColor),
_duration, true, true).show();

// No leak
Toasty.Config.getInstance().allowQueue(true).apply();
Toasty.custom(_context, _text,
ContextCompat.getDrawable(_context, R.drawable.ic_alert_white_36dp),
ContextCompat.getColor(_context, R.color.warn_background),
ContextCompat.getColor(_context, R.color.defaultTextColor),
_duration, true, true).show();
```