travis: Install libnuma dependency for DPDK.

libnuma is a default dependency for DPDK 17.11 because
CONFIG_RTE_EAL_NUMA_AWARE_HUGEPAGES and CONFIG_RTE_LIBRTE_VHOST_NUMA
are enabled by default for most architectures.
libnuma-dev package installation fixes the DPDK build:

	eal_memory.c:56:18: fatal error:
		numa.h: No such file or directory

CC: Mark Kavanagh <mark.b.kavanagh@intel.com>
Fixes: 5e925cc ("netdev-dpdk: DPDK v17.11 upgrade")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ian Stokes <ian.stokes@intel.com>
Tested-by: Ian Stokes <ian.stokes@intel.com>

.travis.yml

@@ -15,6 +15,7 @@ addons:
       - llvm-dev
       - libjemalloc1
       - libjemalloc-dev
+      - libnuma-dev
       - python-sphinx
 
 before_install: ./.travis/${TRAVIS_OS_NAME}-prepare.sh