Set TRAVIS_CMD explicitely, stop using the DEBUG trap

Motivated by the fact that https://github.com/travis-ci/travis-build/pull/199 stopped considering simpler solutions because of the DEBUG trap messing with them.
@drogus basically, `raw` inserts a `Node`, while `cmd` inserts a `Cmd` instance. I guess the options `assert: false, echo: false` could be removed from that line.