use if field to simplify branch exclusion

ref.

https://docs.travis-ci.com/user/conditional-builds-stages-jobs/
https://docs.travis-ci.com/user/conditions-v1

Author: shinnn

.travis.yml

@@ -1,6 +1,5 @@
+if: branch !~ ^v\d
 dist: xenial
-branches:
-  except: /^v\d/
 language: node_js
 node_js: node
 after_script: node_modules/.bin/nyc report | npx coveralls