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

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)