Vienna Convention on the Law of Treaties

We can withdraw from the Lisbon Treaty within 3 months, without bothering with article 50.

From here

In VCLT Art. 54 (a) there is also an important ‘or’ clause regarding exit. In particular, VCLT provides an alternative exit route from a Treaty if there is a ‘fundamental change of circumstances’ (Art.62(1)(a). In such an event the treaty envisages a normal minimum period of notice of withdrawal of three month, but less in exceptional circumstances. (VCLT Art. 65 (2)).

The fundamental change of circumstances is the Leave Vote.