Fix Travis linter warnings

Author: bbc2

.travis.yml

@@ -1,5 +1,6 @@
 language: c
-sudo: false
+os: linux
+dist: xenial
 services:
   - docker
 install: wget https://raw.githubusercontent.com/ocaml/ocaml-ci-scripts/master/.travis-docker.sh
@@ -8,7 +9,7 @@ env:
   global:
     - PACKAGE=zxcvbn
     - DISTRO=debian-stable
-  matrix:
+  jobs:
     - OCAML_VERSION=4.04
     - OCAML_VERSION=4.05
     - OCAML_VERSION=4.06