Add Ruby 3.0 to CI (#337)

Merge pull request 337

.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