(maint) Update and reformat travis.yml

* Reformat YAML
* address build config validation warnings
  * remove outdated config options
  * specify modern build environment

Author: DavidS

.travis.yml

@@ -1,8 +1,8 @@
 ---
+os: linux
+dist: xenial
 language: ruby
 
-sudo: false
-
 bundler_args: --without development acceptance
 
 notifications:
@@ -11,31 +11,27 @@ notifications:
 before_install:
   - gem update bundler
 
-matrix:
+script: 'SPEC_OPTS="--format documentation" COVERAGE="yes" bundle exec rake $CHECK'
+
+jobs:
   fast_finish: true
   include:
-  - rvm: 2.1.9
-    env: PUPPET_GEM_VERSION="~> 4" CHECK=spec
-  - rvm: 2.4.1
-    env: PUPPET_GEM_VERSION="~> 4" CHECK=spec
-  - rvm: 2.4.1
-    env: PUPPET_GEM_VERSION="~> 5" CHECK=spec
-  - rvm: 2.5
-    env: PUPPET_GEM_VERSION="~> 6" CHECK=spec
-  - rvm: 2.1.9
-    env: PUPPET_GEM_VERSION="~> 4" CHECK=rubocop
-  -
-    bundler_args: ""
-    dist: trusty
-    rvm: 2.5
-    before_script:
-    - bundle exec rake 'litmus:provision[docker, centos:7]'
-    - bundle exec rake 'litmus:install_agent[puppet6]'
-    - bundle exec rake 'litmus:install_modules_from_directory[./spec/fixtures/acceptance/modules]'
-    - bundle exec rake litmus:install_gems
-    script:
-    - bundle exec rake litmus:acceptance:parallel
-    services: docker
-    sudo: required
-
-script: 'SPEC_OPTS="--format documentation" COVERAGE="yes" bundle exec rake $CHECK'
+    - rvm: 2.1.9
+      env: PUPPET_GEM_VERSION="~> 4" CHECK=spec
+    - rvm: 2.4.1
+      env: PUPPET_GEM_VERSION="~> 4" CHECK=spec
+    - rvm: 2.4.1
+      env: PUPPET_GEM_VERSION="~> 5" CHECK=spec
+    - rvm: 2.5
+      env: PUPPET_GEM_VERSION="~> 6" CHECK=spec
+    - rvm: 2.1.9
+      env: PUPPET_GEM_VERSION="~> 4" CHECK=rubocop
+    - bundler_args: ""
+      rvm: 2.5
+      before_script:
+        - bundle exec rake 'litmus:provision[docker, centos:7]'
+        - bundle exec rake 'litmus:install_agent[puppet6]'
+        - bundle exec rake 'litmus:install_modules_from_directory[./spec/fixtures/acceptance/modules]'
+        - bundle exec rake litmus:install_gems
+      script:
+        - bundle exec rake litmus:acceptance:parallel