travis: Cache DPDK build.

travis: Cache DPDK build.

This change enables cache for DPDK build directory, so we'll never
build same version of DPDK again. This speeds up each DPDK related
job by 4-6 minutes effectively saving 30-50 minutes of the total time.
Ex. Full TravisCI run on 'trusty' images:

  Without cache:
    Ran for 1 hr 9 min 29 sec
    Total time 4 hrs 55 min 13 sec

  With populated cache:
    Ran for 1 hr 2 min 18 sec
    Total time 4 hrs 20 min 9 sec

Saved:
  Real time: ~7 minutes.
  Total worker time: ~35 minutes.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Aaron Conole <aconole@redhat.com>