As this blog post suggests, there seems to be a hole in the D&D Basic rules. Maybe. The rules for readying an action state:
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
This seems to imply that it is impossible to interrupt a spell in 5e. If you set your trigger to be “I attack the Wizard when he casts a spell”, then you make the attack after the spell has been cast. Does this mean that spells can no longer be interrupted in combat like they could be in previous editions?