Allow failures for ruby 3.0 and head on macOs

https://travis-ci.community/t/ruby-3-0-on-macos-does-not-install/11020

Author: hurricup

.travis.yml

@@ -16,11 +16,16 @@ rvm:
 
 matrix:
   exclude:
-  - os: osx
-    rvm: 2.0
-  - os: osx
-    rvm: 2.1
-  - os: osx
-    rvm: 2.2
-  - os: osx
-    rvm: 2.3
+    - os: osx
+      rvm: 2.0
+    - os: osx
+      rvm: 2.1
+    - os: osx
+      rvm: 2.2
+    - os: osx
+      rvm: 2.3
+  allow_failures:
+    - os: osx
+      rvm: 3.0
+    - os: osx
+      rvm: ruby-head