ci: enable ccache in Travis

We try to make the planet happy travis builds dpdk.

Also, the 'sudo' flag was recently deprecated.  Drop it.

Suggested-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Luca Boccassi <bluca@debian.org>

.travis.yml

@@ -1,4 +1,5 @@
 language: c
+cache: ccache
 compiler:
   - gcc
   - clang
@@ -24,8 +25,6 @@ extra_packages: &extra_packages
 
 before_install: ./.ci/${TRAVIS_OS_NAME}-setup.sh
 
-sudo: false
-
 env:
   - DEF_LIB="static"
   - DEF_LIB="shared"