databases/pecl-cassandra: add portepoch

In 9967c437a2b52c64533fb17d3e33772c8a3f9daa the port got updated
from 1.3.2 to 1.3.2.a.20200527, which is a lower version number.

Add portepoch accordingly.

Differential Revision: https://reviews.freebsd.org/D34006
Approved by: rene