Correct travis.yml to only build extended tests when explicitely asked to

The check for this was done by checking if $TRAVIS_EVENT_TYPE is
"pull_request".  The trouble is that when new data is pushed to an
already existing pull request, the event type is "push".

Better then to go with another documented variable, $TRAVIS_PULL_REQUEST,
which is "false" in non-PR builds.

Ref: https://docs.travis-ci.com/user/environment-variables/#Default-Environment-Variables



Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3205)