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

(cherry picked from commit 6df44cf65fbc7e150965149d7e681ac3e22d11d8)

.travis.yml

@@ -152,7 +152,7 @@ matrix:
 before_script:
     - env
     - if [ "$TRAVIS_PULL_REQUEST" != "false" -a -n "$EXTENDED_TEST" ]; then
-          (git log -1 $TRAVIS_COMMIT_RANGE | grep '\[extended tests\]' > /dev/null) || exit 0;
+          (git log -1 $TRAVIS_COMMIT_RANGE | grep '\[extended tests\]' > /dev/null) || travis_terminate 0;
       fi
     - if [ -n "$DESTDIR" ]; then
           sh .travis-create-release.sh $TRAVIS_OS_NAME;