Fix default encoding in config.ru

Ruby 1.9 cares about encoding, and apache starts up as US-ASCII in most
cases.  If you have a manifest that is UTF-8, then puppet won't be able to
handle it.  To work around that, we change the default encoding.  See
https://tickets.puppetlabs.com/browse/PUP-1386 and
https://github.com/puppetlabs/puppet/pull/1666 for more details.