travis: enable apt update.

Change-Id: I102b669238dcbf3b8fce8b7429da08f471a20811
Reviewed-on: https://code.wireshark.org/review/27575
Reviewed-by: Dario Lombardo <lomato@gmail.com>

.travis.yml

@@ -17,6 +17,7 @@ matrix:
     compiler: gcc
 addons:
   apt:
+    update: true
     sources:
     - ubuntu-toolchain-r-test
     - llvm-toolchain-trusty-5.0