From the Upstart cookbook, Changing the Default Shell. 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