Make sensu services more robust

Without this change I get this error in my server:

```
---- Begin output of /etc/init.d/sensu-dashboard restart ----
STDOUT: * Stopping sensu-dashboard
   ...fail!
 * Starting sensu-dashboard
   ...fail!
STDERR: /var/lib/gems/1.9.1/gems/bundler-1.5.1/lib/bundler/rubygems_integration.rb:240:in `block in replace_gem': sensu-dashboard is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
        from /opt/sensu/bin/sensu-dashboard:22:in `<main>'
---- End output of /etc/init.d/sensu-dashboard restart ----
```