Run tests on Ruby 3.0 (#390)

Rails 5.2 and below [do not work with Ruby 3][rails issue], so don't run
those test

[rails issue]: https://github.com/rails/rails/issues/40938.

.travis.yml

@@ -6,6 +6,7 @@ rvm:
   - 2.5
   - 2.6
   - 2.7
+  - 3.0
   - ruby-head
 branches:
   only:
@@ -20,6 +21,13 @@ gemfile:
   - gemfiles/rails6.0.gemfile
   - gemfiles/rails6.1.gemfile
 matrix:
+  exclude:
+    - rvm: 3.0
+      gemfile: gemfiles/rails5.0.gemfile
+    - rvm: 3.0
+      gemfile: gemfiles/rails5.1.gemfile
+    - rvm: 3.0
+      gemfile: gemfiles/rails5.2.gemfile
   fast_finish: true
   allow_failures:
     - rvm: ruby-head