Fix up bundler version issue

Author: dlackty

.travis.yml

@@ -1,8 +1,10 @@
 language: ruby
 
 before_install:
-  # Workaround for https://github.com/travis-ci/travis-ci/issues/8969
-  - gem update --system
+  # Source: <https://docs.travis-ci.com/user/languages/ruby/#bundler-20>
+  - gem update --system '2.7.8' --no-document
+  - gem uninstall -v '>= 2' -i $(rvm gemdir)@global -ax bundler || true
+  - gem install bundler -v '< 2' --no-document
 
 rvm:
   - 2.4
@@ -19,13 +21,15 @@ env:
 
 matrix:
   exclude:
-    - rvm: 2.5
-      env: RAILS_VERSION=4.2
-    - rvm: 2.5
-      env: RAILS_VERSION=5.0
+    - rvm: 2.4
+      env: RAILS_VERSION=6.0
     - rvm: 2.6
       env: RAILS_VERSION=4.2
     - rvm: 2.6
       env: RAILS_VERSION=5.0
+    - rvm: 2.7
+      env: RAILS_VERSION=4.2
+    - rvm: 2.7
+      env: RAILS_VERSION=5.0
 
 sudo: false