Work around RubyGems TTY detection issue (#2234)

rubygems/rubygems#3036 (comment)

Author: pirj

script/update_rubygems_and_install_bundler

@@ -6,7 +6,7 @@ set -e
 source script/functions.sh
 
 if is_ruby_23_plus; then
-  gem update --system
+  yes | gem update --system
   gem install bundler
 else
   echo "Warning installing older versions of Rubygems / Bundler"