Jump to last bundler version how doesn't break on TravisCI

On bundle 1.16.3 we start seeing fail builds:
- rspec#2013 (comment)

This is related to:
- rspec#2013 (comment)

As mentionned this has been fixed in 1.16.4:
- rubygems/bundler#6629 (comment)

.travis.yml

@@ -24,7 +24,7 @@ bundler_args: "--binstubs --path ../bundle --retry=3 --jobs=3"
 
 before_install:
   - gem update --system
-  - gem install bundler -v 1.16.2
+  - gem install bundler
   - script/clone_all_rspec_repos
 
 before_script: