Stop doing bundle install in Travis builds
Previously this was needed when the ruby script included Foreman itself. Now it's standalone and only requires JSON. That greatly reduces the setup time.
Fixes #23643 - Stop bundle install in Travis
Since 90db5e364ff209f093c64849e21d3362644367f8 we no longer require core
Foreman in script/plugin_webpack_directories.rb which means we no longer
need all dependencies installed. Just Ruby with JSON support. This
should greatly reduce the setup time on Travis.
We assume that if no Gemfile.lock is present, it's a clean clone
without any plugins. Then we can safely ignore incomplete bundles.