Remove null_safety branch from .travis.yml

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index cc94606..32b9f13 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -28,7 +28,7 @@
 
 # Only building master means that we don't run two builds for each pull request.
 branches:
-  only: [master, null_safety]
+  only: [master]
 
 cache:
   directories: