Merge pull request #31029 from yahonda/ignore_uninstall_bundler_at_ruby_head

Ignore "gem "bundler" cannot be uninstalled because it is a default gem" error when tested with ruby-head

.travis.yml

@@ -28,7 +28,7 @@ bundler_args: --without test --jobs 3 --retry 3
 before_install:
   - "rm ${BUNDLE_GEMFILE}.lock"
   - "travis_retry gem update --system"
-  - "rvm @global do gem uninstall bundler --all --ignore-dependencies --executables"
+  - "rvm @global do gem uninstall bundler --all --ignore-dependencies --executables || true"
   - "travis_retry gem install bundler -v 1.15.4"
   - "[ -f /tmp/beanstalkd-1.10/Makefile ] || (curl -L https://github.com/kr/beanstalkd/archive/v1.10.tar.gz | tar xz -C /tmp)"
   - "pushd /tmp/beanstalkd-1.10 && make && (./beanstalkd &); popd"