Add Ruby3 preview1 to build matrix with allow_failures

Author: benoittgt

travis/.travis.yml

@@ -23,6 +23,7 @@ rvm:
   - 2.5.8
   - 2.6.6
   - 2.7.1
+  - ruby-3.0.0-preview1
   - ruby-head
   - ree
   - rbx-3
@@ -38,6 +39,7 @@ matrix:
   allow_failures:
     - rvm: jruby-head
     - rvm: ruby-head
+    - rvm: ruby-3.0.0-preview1
     - rvm: rbx-3
   fast_finish: true
 branches: