Allow double quotes in config values

For example, if one wanted to store JSON in a config value (granted, not a great idea in general).

Setting `{"foo":"bar"}` via `ssh -t ... config:set <app> TEST='{"foo":"bar"}'` would result in the following (broken) line in ENV:

    export TEST={"foo":"bar"}

With this pull request the value is strong quoted

    export TEST='{"foo":"bar"}'

For comparison, double quotes in config values do work on Heroku.

/cc @mdpye, thanks for the digging!