Add Ruby 3.0 to CI

jekyll-feed will be [included in Fedora 34][pkg] which will also use [Ruby 3.0
as system Ruby interpreter][ruby]. This combination is known to work, so it
would make sense to add it to CI.

[pkg]: https://src.fedoraproject.org/rpms/rubygem-jekyll-feed
[ruby]: https://fedoraproject.org/wiki/Changes/Ruby_3.0

.travis.yml

@@ -1,16 +1,16 @@
 language: ruby
 cache: bundler
 rvm:
-  - &latest_ruby 2.7
+  - 3.0
+  - 2.7
   - 2.5
 env:
   matrix:
     - JEKYLL_VERSION="~> 3.9"
+    - JEKYLL_VERSION="~> 4.2"
 matrix:
-  include:
-    - rvm: *latest_ruby
-      env: JEKYLL_VERSION="~> 3.9"
-    - rvm: *latest_ruby
+  exclude:
+    - rvm: 2.5
       env: JEKYLL_VERSION="~> 4.2"
 before_install:
 - gem update --system