CI. Replace include rules with exclude ones

Author: andrykonchin

.travis.yml

@@ -14,34 +14,23 @@ gemfile:
   - gemfiles/rails_5_0.gemfile
   - gemfiles/rails_5_1.gemfile
   - gemfiles/rails_5_2.gemfile
-  - gemfiles/rails_5_2.gemfile
+  - gemfiles/rails_6_0.gemfile
+  - gemfiles/rails_6_1.gemfile
 
+# https://docs.travis-ci.com/user/build-matrix
 matrix:
-  include:
-    # Rails 6.0
-    # It requires Ruby 2.5 and above
-    - rvm: jruby-9.2.14.0
-      gemfile: gemfiles/rails_6_0.gemfile
-    - rvm: ruby-2.5.3
-      gemfile: gemfiles/rails_6_0.gemfile
-    - rvm: ruby-2.6.3
-      gemfile: gemfiles/rails_6_0.gemfile
-    - rvm: ruby-2.7.0
+  exclude:
+
+    # Rails 6.0 requires Ruby 2.5 and above
+    - rvm: ruby-2.3.8
       gemfile: gemfiles/rails_6_0.gemfile
-    - rvm: ruby-3.0.0
+    - rvm: ruby-2.4.6
       gemfile: gemfiles/rails_6_0.gemfile
 
-    # Rails 6.1
-    # It requires Ruby 2.5 and above
-    - rvm: jruby-9.2.14.0
-      gemfile: gemfiles/rails_6_1.gemfile
-    - rvm: ruby-2.5.3
-      gemfile: gemfiles/rails_6_1.gemfile
-    - rvm: ruby-2.6.3
-      gemfile: gemfiles/rails_6_1.gemfile
-    - rvm: ruby-2.7.0
+    # Rails 6.1 requires Ruby 2.5 and above
+    - rvm: ruby-2.3.8
       gemfile: gemfiles/rails_6_1.gemfile
-    - rvm: ruby-3.0.0
+    - rvm: ruby-2.4.6
       gemfile: gemfiles/rails_6_1.gemfile
 
 ### BUILD LIFECYCLE STEPS ###