Uses a config fixture file to enable testing

A config file located in the ambiguous `spec/brightbox/config` was
required to enable the specs to run against the VCR recordings.

This uses a new fixture in `spec/fixtures/config_files/config` to enable
anyone to run the tests without knowing the undocumented steps required.

The rarer case, using a live connection to record traffic, is not yet
covered.
Updates Rake to v10.0.4 to silence warning

Rake 0.8.7 was warning about the use of Config