.travis.yml: Print TRAVIS_* environment variables
.travis.yml: Print TRAVIS_* environment variables

To make make debugging Travis environment issues more straightforward.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

vbatts/oci-image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec,opencontainers/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec