Use bundle exec to run delayed jobs

When installing Consul Democracy 2.2.0, we were getting an error:

```
`require': cannot load such file -- sassc-embedded (LoadError)
from <internal:rubygems/core_ext/kernel_require.rb>:86:in `require'
from config/application.rb:1:in
```

No idea why this gem isn't detected when running `bin/delayed_job`.
When deploying with Capistrano, we use `bundle exec bin/delayed_job`,
and that works fine, so we're using it here as well.

roles/queue/tasks/main.yml

@@ -1,5 +1,5 @@
 - name: Start DelayedJobs queue
-  shell: "{{ rvm_command }} && RAILS_ENV={{ env }} EXECJS_RUNTIME=Disabled bin/delayed_job -m -n 2 restart"
+  shell: "{{ rvm_command }} && RAILS_ENV={{ env }} EXECJS_RUNTIME=Disabled bundle exec bin/delayed_job -m -n 2 restart"
   args:
     executable: /bin/bash
     chdir: "{{ release_dir }}"