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.

To change the format back to the default for a particular logger, you can
use `my_logger = Logger::Formatter.new`.
Merge branch 'issue353-log-format-change' of github.com:rhunter/grape
Merged #354 and added a spec.