ci.yml in enum_ish-1.4.1

- old
+ new

@@ -2,10 +2,10 @@
 
 on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: false
       matrix:
         ruby: [2.3, 2.4, 2.5, 2.6, 2.7, 3.0]
         gemfile: ['rails50', 'rails51', 'rails52', 'rails60', 'rails61']