Invalidate the intrinsicContentSize

Was getting wonky layout issues when using TTTAttributedLabel in my UITableViewCell subclass that uses Auto Layout

The implementation of setAttributedText: should call invalidateIntrinsicContentSize per the docs:
```
Call this when something changes in your custom view that invalidates its intrinsic content size. This allows the constraint-based layout system to take the new intrinsic content size into account in its next layout pass.
```