finatra: Add scala 2.13 in Travic CI config
finatra: Add scala 2.13 in Travic CI config

Problem

We are pulling in 2.13 cross-building targets, currently, no build on OSS against
2.13.

Solution

Add scala 2.13.1 in Travis CI config. Mark them allow failures so not updated
targets failures won't give us notifications.

JIRA Issues: CSL-10135

Differential Revision: https://phabricator.twitter.biz/D545401

twitter/finatra,twitter/finatra,twitter/finatra