(maint) Use Ruby 2.5 on Travis for PDK as a library tests

Concurrent-ruby 1.1.6 has a strange thread timing issue under Ruby 2.4 and the
spec/unit/pdk/analytics/util_spec.rb file. Causing facter to error with
"cannot alloc thread" which then halts the test. It doesn't occur on Appveyor
(Windows) or the regular rspec under Ruby 2.4 testing. So just use Ruby 2.5
instead.

.travis.yml

@@ -1,4 +1,7 @@
-sudo: false
+---
+os:
+  - linux
+dist: xenial
 language: ruby
 bundler_args: "--without development"
 script:
@@ -15,7 +18,7 @@ before_install:
       gem install bundler --no-document;
     fi
 cache: bundler
-matrix:
+jobs:
   include:
     - rvm: 2.4.3
       env: CHECK=rubocop
@@ -35,7 +38,10 @@ matrix:
       env: CHECK=spec
     - rvm: 2.1.9
       env: CHECK=spec BUNDLER_VERSION=1.17.3
-    - rvm: 2.4.3
+    # concurrent-ruby 1.1.6 has a strange thread timing issue under Ruby 2.4 and the
+    # spec/unit/pdk/analytics/util_spec.rb file. It doesn't occur on Appveyor (Windows)
+    # or the regular rspec under Ruby 2.4 testing. So just use Ruby 2.5 instead.
+    - rvm: 2.5
       env: CHECK=test_pdk_as_library
 branches:
   only: