Travis CI configuration updates

This is trying to work around a couple dependency issues. Rails 4.2
depends on bundler < 2.0 so we're trying to lock to that along with
installing Bundler 1.x. Rails 6.0 requires Ruby 2.5+ and so we need to
avoid testing it on Ruby 2.3 and 2.4.

.travis.yml

@@ -1,30 +1,56 @@
-after_success:
-  - bundle exec codeclimate-test-reporter
-before_install:
-  - gem update bundler rake
-branches:
-  only:
-    - master
+dist: bionic
+language: ruby
+
+rvm:
+  - "2.3"
+  - "2.4"
+  - "2.5"
+  - "2.6"
+  - ruby-head
+
 gemfile:
-  - gemfiles/rails42.gemfile
+  - Gemfile
   - gemfiles/rails50.gemfile
   - gemfiles/rails51.gemfile
   - gemfiles/rails52.gemfile
   - gemfiles/rails60.gemfile
-  - gemfiles/rails-stable.gemfile
-language: ruby
+
 matrix:
+  include:
+    # Rails 4.2 requires Bundler 1.x which was being difficult to use with
+    # Travis and Ruby 2.6+ since Bundler is now packaged with Ruby.
+    - rvm: "2.3"
+      gemfile: gemfiles/rails42.gemfile
+    - rvm: "2.4"
+      gemfile: gemfiles/rails42.gemfile
+    - rvm: "2.5"
+      gemfile: gemfiles/rails42.gemfile
+
+  exclude:
+    - rvm: "2.3"
+      gemfile: Gemfile
+    - rvm: "2.3"
+      gemfile: gemfiles/rails60.gemfile
+    - rvm: "2.4"
+      gemfile: Gemfile
+    - rvm: "2.4"
+      gemfile: gemfiles/rails60.gemfile
   allow_failures:
     - rvm: ruby-head
+
+before_install:
+  # Use bundler 1.x with Rails 4.2
+  - "([[ $BUNDLE_GEMFILE =~ rails42 ]] && (gem uninstall -v '>= 2' -ax bundler || true) && gem install bundler -v '< 2') || true"
+
+script:
+  - bundle exec rspec --format=documentation
+
+# Report test cover to codeclimate
+after_success:
+  - bundle exec codeclimate-test-reporter
+
 notifications:
   webhooks:
     on_start: always
     urls:
       - https://buildlight.collectiveidea.com/
-rvm:
-  - "2.3"
-  - "2.4"
-  - "2.5"
-  - ruby-head
-script: bundle exec rspec --format=documentation
-sudo: false