.travis.yml: re-enable builds on master branch, disable building on branch push in travis settings

https://travis-ci.org/github/awesome-selfhosted/awesome-selfhosted/settings
only leave "Build Pull Requests" enabled - only run Travis on PRs and from weekly cron job

.travis.yml

@@ -1,9 +1,5 @@
 language: node_js
 
-branches:
-  except:
-  - master
-
 node_js:
   - "node"