Update based on default xenial distro.

Signed-off-by: Lantao Liu <lantaol@google.com>

.travis.yml

@@ -21,16 +21,14 @@ cache:
     - "${HOME}/google-cloud-sdk/"
 
 before_install:
-  # libseccomp in trusty is not new enough, need backports version.
-  - sudo sh -c "echo 'deb http://archive.ubuntu.com/ubuntu trusty-backports main restricted universe multiverse' > /etc/apt/sources.list.d/backports.list"
   - sudo apt-get update
   # Enable ipv6 for dualstack integration test.
   - sudo sysctl net.ipv6.conf.all.disable_ipv6=0
 
 install:
   - sudo apt-get install btrfs-tools
-  - sudo apt-get install libseccomp2/trusty-backports
-  - sudo apt-get install libseccomp-dev/trusty-backports
+  - sudo apt-get install libseccomp2
+  - sudo apt-get install libseccomp-dev
   - sudo apt-get install socat
 
 before_script: