.travis.yml in in_threads-1.5.0 vs .travis.yml in in_threads-1.5.1

@@ -3,25 +3,28 @@
 rvm:
   - '1.8.7-p371'
   - '1.9.3-p551'
   - '2.0.0-p648'
   - '2.1.10'
-  - '2.2.8'
-  - '2.3.5'
-  - '2.4.2'
+  - '2.2.10'
+  - '2.3.8'
+  - '2.4.5'
+  - '2.5.3'
+  - '2.6.0'
   - 'ruby-head'
   - 'jruby-9.0.5.0'
   - 'jruby-9.1.9.0'
 before_install:
+  - gem install bundler
+  - ruby -e 'require "rubygems"; require "bundler/inline"; gemfile{ source "https://rubygems.org"; gem "rubygems-update" }' # get compatible version of rubygems-update
   - gem update --system
-  - gem update bundler
 script: bundle exec rspec
 matrix:
   include:
     - env: RUBOCOP=✓
-      rvm: '2.4.2'
+      rvm: '2.5.3'
       script: bundle exec rubocop
     - env: CHECK_RUBIES=✓
-      rvm: '2.4.2'
+      rvm: '2.5.3'
       script: bundle exec travis_check_rubies
   allow_failures:
     - rvm: 'ruby-head'