Update .travis.yml

Author: kenyonj

.travis.yml

@@ -4,7 +4,9 @@ branches:
   only:
     - master
 bundler_args: --jobs=3 --retry=3
-before_install: ./script/setup
+before_install:
+  - gem install bundler -v 1.16.1
+  - ./script/setup
 script: ./script/cibuild
 notifications:
   email: false