Warn user when log channels drop history.

Certain channels get flagged into this due to excessive output; this
will alert the user that this has occurred.
Read general-purpose warnings from tail sessions.
Actually handle errors in log tailing.
Merge pull request #1044 from heroku/log-history-warning

Log history warning
v3.4.3
do not try to fork to the same app
confirm before deleting app
Use standard confirm_command to check app name in confirm
update Gemfile.lock
move to MultiJson.
remove vendored okjson.
remove ok json from specs.
Adds an OkJson shim with deprecation warning

This ensures a clean deprecation and migration path
for plugins that were depending on Heroku::OkJson directly.
Require multi_json dependency
don't use MultiJson directly in certs.
Merge pull request #887 from heroku/multi_json

Multi json
Merge pull request #1045 from heroku/confirm-before-deleting-fork

A mal-formed fork command can delete your application