Fix bug in comment requirement

Previously, running a cap command like:

```
cap env deploy -s comment_value=foobar
```

would not work, as comment_value would error out with a LocalJumpError.
This patch fixes that issue by properly wrapping the code inside a conditional.

Tested with:

```
cap env deploy #interactive
cap env deploy -s comment_value=foobar #noninteractive
```

successfully.

(Reminder to commenters: this is a public repo)

CC @dougbarth @steverice