Exec the command in upstart

In order to ensure that upstart tracks the correct PID, make sure we
exec to the command requested.

Fixes nickstenning#131.
Closes nickstenning#144.

honcho/export/templates/upstart/process.conf

@@ -6,4 +6,4 @@ exec su - {{ user }} -s {{ shell }} -c 'cd {{ app_root }};
 {%- for k, v in process.env.items() -%}
   export {{ k }}={{ v | shellquote }};
 {%- endfor -%}
-{{ process.cmd }} >> {{ log }}/{{ process.name|dashrepl }}.log 2>&1'
+exec {{ process.cmd }} >> {{ log }}/{{ process.name|dashrepl }}.log 2>&1'