Remove deleted null_safety branch from Travis (#100)

diff --git a/.travis.yml b/.travis.yml
index b9d1117..f72e622 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -29,7 +29,7 @@
 
 # Only building master means that we don't run two builds for each pull request.
 branches:
-  only: [master, null_safety]
+  only: [master]
 
 cache:
   directories: