travis: Use default Maven settings

Throw away Travis-CI's custom Maven settings, because they are causing
massive CI failures when Maven slows to a crawl/hangs because of
failures contaicting repository.apache.org.

Travis-CI's settings includes repo.maven.apache.org, oss.sonatype
(releases and snapshots), and repository.apache.org (releases and
snapshots). Now we will just be using Maven's default, which may just
be repo.maven.apache.org.