ci: try bumping more Travis jobs to Ansible 2.8.

.travis.yml

@@ -49,7 +49,7 @@ matrix:
       env: MODE=debops_common VER=2.4.6.0
     # 2.5.7; 3.6 -> 2.7
     - python: "3.6"
-      env: MODE=debops_common VER=2.6.2
+      env: MODE=debops_common VER=2.8.0
 
     # ansible_mitogen tests.
 
@@ -61,14 +61,14 @@ matrix:
     - python: "2.6"
       env: MODE=ansible VER=2.4.6.0
     - python: "2.6"
-      env: MODE=ansible VER=2.6.2
+      env: MODE=ansible VER=2.8.0
 
     # 3.6 -> {debian, centos6, centos7}
     - python: "3.6"
       env: MODE=ansible VER=2.4.6.0
     - python: "3.6"
-      env: MODE=ansible VER=2.6.2
+      env: MODE=ansible VER=2.8.0
 
     # Sanity check against vanilla Ansible. One job suffices.
     - python: "2.7"
-      env: MODE=ansible VER=2.6.2 DISTROS=debian STRATEGY=linear
+      env: MODE=ansible VER=2.8.0 DISTROS=debian STRATEGY=linear