ci: fix Travis config warnings

ci: fix Travis config warnings

Reading https://config.travis-ci.com/ and using
https://config.travis-ci.com/explore to check changes, we can cleanup
some warnings reported by the config validation options in Travis.

Example on a job in master:
https://travis-ci.com/DPDK/dpdk/builds/149537002/config

Build config validation:
root: deprecated key doc_packages (anchor on a non-private key)
root: deprecated key build_32b_packages (anchor on a non-private key)
root: deprecated key libabigail_build_packages (anchor on a non-private key)
root: deprecated key extra_packages (anchor on a non-private key)
root: deprecated key aarch64_packages (anchor on a non-private key)
root: key matrix is an alias for jobs, using jobs

For the "(anchor on a non-private key)" warnings, the Travis
documentation [1] recommends prefixing private keys with _ (Travis
schema is available at [2]).

The use of the "matrix" key is allowed, but it is just an alias to the
"jobs" key.

1: https://docs.travis-ci.com/user/build-config-yaml#private-keys-as-yaml-anchors-and-aliases-and-external-tooling
2: https://github.com/travis-ci/travis-yml/blob/730a77f4028b91293a55c8c1c52fc72e64379dde/schema.json

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>