Fixes supervisord export not properly quoting environment values.

Hey there,

I've been playing around with `foreman export supervisord` and noticed that supervisord crashes with environment values that aren't properly quoted.

Per http://supervisord.org/configuration.html:

> environment:
>> Values containing non-alphanumeric characters should be quoted (e.g. KEY="val:123",KEY2="val,456").

This patch always quotes the value. What do you think?

Thanks!