Silence deprecation warnings.

The remaining deprecation warnings are for the plugins, and they
interfere with incoming mail handling by producing output on stderr.
Require activesupport before mail gem.

If activesupport isn't present, the mail gem will load it's own version,
and when rails is eventually loaded in this script, the warning
"warning: already initialized constant VALID CHARACTER" is written to
standard error.
Add a dummy exception notification address for testing

ActionMailer now checks for a 'to' address on sending mail, so supply
one so that we can check exception notification mail sending. Also check
that we have one before trying to call the exception notification code.