Bundler executable not available

I'm having a problem with running bundler. The deploy completes `deploy:symlink:linked_dirs` and then fails on `bundler:install`:

```
01 RBENV_ROOT=$HOME/.rbenv RBENV_VERSION=2.4.2 $HOME/.rbenv/bin/rbenv exec bundle install --path /home/ubuntu/rails//current/bin/bundle --without development test --deployment --quiet --binstubs
01 rbenv: bundle: command not found
```

I've tried installing it manually with `rbenv exec gem install bundler` and `rbenv rehash`, but the executable `bundle` does not appear.

`capistrano 3.10.0`
`capistrano-rbenv 2.1.3`
`capistrano-bundler 1.3.0`
`capistrano-rails 1.3.0`
`rails 4.2.10`
`rbenv 1.1.1`

Start at the bundler:install deployment step and reproduce the failure with the listed Capistrano, capistrano-rbenv, capistrano-bundler, Rails, and rbenv versions. Check how the rbenv-managed Ruby exposes the bundle executable after installation and rehashing. Done means identifying a reproducible cause and confirming that deployment can complete the bundler:install step.

Written by the indexing model from the issue text.