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: 730a77f402/schema.json

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

.travis.yml

@@ -19,22 +19,22 @@ addons:
     packages: &required_packages
       - [libnuma-dev, python3-setuptools, python3-wheel, python3-pip, ninja-build]
 
-aarch64_packages: &aarch64_packages
+_aarch64_packages: &aarch64_packages
   - *required_packages
   - [gcc-aarch64-linux-gnu, libc6-dev-arm64-cross, pkg-config-aarch64-linux-gnu]
 
-extra_packages: &extra_packages
+_extra_packages: &extra_packages
   - *required_packages
   - [libbsd-dev, libpcap-dev, libibverbs-dev, libcrypto++-dev, libfdt-dev, libjansson4]
 
-libabigail_build_packages: &libabigail_build_packages
+_libabigail_build_packages: &libabigail_build_packages
   - [autoconf, automake, libtool, pkg-config, libxml2-dev, libdw-dev]
 
-build_32b_packages: &build_32b_packages
+_build_32b_packages: &build_32b_packages
   - *required_packages
   - [gcc-multilib]
 
-doc_packages: &doc_packages
+_doc_packages: &doc_packages
   - [doxygen, graphviz, python3-sphinx]
 
 before_install: ./.ci/${TRAVIS_OS_NAME}-setup.sh
@@ -44,7 +44,7 @@ env:
   - DEF_LIB="shared"
   - DEF_LIB="shared" RUN_TESTS=1
 
-matrix:
+jobs:
   include:
   - env: DEF_LIB="static" BUILD_32BIT=1
     compiler: gcc