travis: allow all branches except travis.tmp to be built

Author: matthiaskrgr

.travis.yml

@@ -10,13 +10,9 @@ os:
 sudo: false
 
 branches:
-  only:
-    # This is where pull requests from "bors r+" are built.
-    - staging
-    # This is where pull requests from "bors try" are built.
-    - trying
-    # Also build pull requests.
-    - master
+  skip:
+    # Used by bors
+    - trying.tmp
 
 env:
  global: