Disable mod_http2 and mod_ssl_ct for prefork builds, since the former shouldn't be used under prefork and the latter isn't tested at all. Possibly related to infrequent prefork child segfaults under pool-debug. e.g. https://travis-ci.org/github/apache/httpd/jobs/736044109 is a multi-threaded prefork child dying with both mod_h2 and mod_ssl_ct threads active.
httpd/httpd/trunk/.travis.yml
httpd/httpd/trunk/test/README.travis