travis: fix DPDK cache
travis: fix DPDK cache

Unfortunately, keys in .travis.yml override each other rather than just
combining. Thus second cache instance (ccache) disabled first one
(DPDK). Fix that by merging both cache instance into single key in
.travis.yml.

Signed-off-by: Dmitry Eremin-Solenikov <05e7612c354c8479a2d1e9b11fb63ce18664b3ca@linaro.org>
Signed-off-by: Maxim Uvarov <db4d16e02ae2d7493db430203537da8b2e34f290@linaro.org>

dkrot/odp,dkrot/odp,dkrot/odp,dkrot/odp