travis: add Bionic (Ubuntu 18.04 LTS) to test matrix

Using bionic (current LTS) as default, and add xenial (Ubuntu 16.04 LTS)
to the matrix, to test the previous LTS release as well on master

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.travis.yml

@@ -1,4 +1,4 @@
-dist: xenial
+dist: bionic
 sudo: required
 # setup travis so that we can run containers for integration tests
 services:
@@ -18,6 +18,22 @@ env:
   - TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runtime.v1.linux TRAVIS_CGO_ENABLED=1
   - TRAVIS_GOOS=darwin TRAVIS_CGO_ENABLED=0
 
+matrix:
+  include:
+    # On master, also test against the previous LTS (Xenial / Ubuntu 16.04 LTS)
+    - if: branch = master
+      os: linux
+      dist: xenial
+      env: TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runc.v1 TRAVIS_CGO_ENABLED=1
+    - if: branch = master
+      os: linux
+      dist: xenial
+      env: TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runc.v2 TRAVIS_CGO_ENABLED=1
+    - if: branch = master
+      os: linux
+      dist: xenial
+      env: TRAVIS_GOOS=linux TEST_RUNTIME=io.containerd.runtime.v1.linux TRAVIS_CGO_ENABLED=1
+
 go_import_path: github.com/containerd/containerd
 
 addons:
@@ -31,7 +47,7 @@ addons:
       - python-minimal
       - libcap-dev
       - libaio-dev
-      - libprotobuf-c0-dev
+      - libprotobuf-c-dev
       - libprotobuf-dev
       - socat