Include "Log-History: none" header when no_redis flag is set.

This will allow the toolbelt to warn users when their apps get flagged
in for excessive output volume.
Turns out lookup_flag returns the flag, not true/false.
Replace no_tail_warning with generalized format_warning.
Use format_warning for no_redis tails.
Revert "Replace no_tail_warning with generalized format_warning."

This reverts commit 8ace66c1a89ad00da21b35dfe6ce56fae7401479.

We don't actually want that formatting for HTTP headers.
Merge pull request #85 from heroku/log-history-warning

Log history warning