travis: use docker, test on 4.08 and 4.09 as well

.travis.yml

@@ -1,14 +1,19 @@
 language: c
-install: wget https://raw.githubusercontent.com/ocaml/ocaml-ci-scripts/master/.travis-opam.sh
-script: bash -ex .travis-opam.sh
-sudo: required
+install: wget https://raw.githubusercontent.com/ocaml/ocaml-ci-scripts/master/.travis-docker.sh
+script: bash -ex .travis-docker.sh
+sudo: false
+services:
+  - docker
 env:
   global:
     - PACKAGE="albatross"
+    - DISTRO=ubuntu
     - TESTS=false
   matrix:
     - OCAML_VERSION=4.05
     - OCAML_VERSION=4.06
     - OCAML_VERSION=4.07
+    - OCAML_VERSION=4.08
+    - OCAML_VERSION=4.09
 notifications:
   email: false