From the Upstart cookbook, [Changing the Default Shell][1]. There are 3 options, the first 2 involve changing your default shell from `/bin/sh` to something else. But the 3rd option looks like it would solve your particular issue.

*excerpt*

> Use a "here document" (assuming your chosen shell supports them)
> within the Job Configuration Files you wish to run with a different
> shell:
> 
> script
> /bin/bash <<EOT
> 
> echo "Hi - I am running under the bash shell"
> 
> date
>
> echo "and so am I :)"
>
> EOT
> end script


 [1]: http://upstart.ubuntu.com/cookbook/#changing-the-default-shell