Lock bundler only for 2.4

Author: dlackty

.travis.yml

@@ -1,11 +1,5 @@
 language: ruby
 
-before_install:
-  # 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
   - 2.5
@@ -20,6 +14,13 @@ env:
   - RAILS_VERSION=6.0
 
 matrix:
+  include:
+    - rvm: 2.4
+      before_install:
+        # 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
   exclude:
     - rvm: 2.4
       env: RAILS_VERSION=6.0