travis: Switch to dpdk master branch.

Make this branch point to current dpdk master branch so that we can
track API breakage.

Note: this should not be merged to master, intended for dpdk-latest
branch only.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>

.travis.yml

@@ -30,7 +30,9 @@ addons:
 
 before_install: ./.travis/${TRAVIS_OS_NAME}-prepare.sh
 
-before_script: export PATH=$PATH:$HOME/bin
+before_script:
+  - export PATH=$PATH:$HOME/bin
+  - export DPDK_VER=refs/heads/master
 
 env:
   - OPTS="--disable-ssl"