Only require the necessary bits of ActiveSupport

Addresses issue #353 -- Log formats silently changed between versions

The default log format changes after `require 'active_support/core_ext/logger'`, and
of course `require active_support/all` brings in everything including loggers.

(If you can't help but include all of ActiveSupport, you can change the log format back to the default for a particular logger, with `my_logger.format = Logger::Formatter.new`.)