Fix exit issue in travisci

Fix exit issue in travisci

Ungraceful 'exit' probably causes unexpeced error on background activity.
So replace 'exit' with recommended 'travis_terminate'. Also see
https://travis-ci.community/t/exit-0-cannot-exit-successfully-on-arm/5731/4

Change-Id: I382bd93a3e15ecdf305bab23fc4adefbf0348ffb

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10561)