chore: use regex to whitelist branches for CI (#12439)

Author: mmalerba

.circleci/config.yml

@@ -69,3 +69,7 @@ general:
   branches:
     only:
       - master
+      # 5.2.x, 6.0.x, etc
+      - /\d+\.\d+\.x/
+      # 5.x, 6.x, etc
+      - /\d+\.x/

.travis.yml

@@ -18,13 +18,10 @@ addons:
 branches:
   only:
   - master
-  - 5.0.x
-  - 5.1.x
-  - 5.2.x
-  - 6.0.x
-  - 6.1.x
-  - 6.2.x
-  - 6.3.x
+  # 5.2.x, 6.0.x, etc
+  - /\d+\.\d+\.x/
+  # 5.x, 6.x, etc
+  - /\d+\.x/
 
 jobs:
   include: