Adds an RSpec Rake task as the default

I find it far more intuituve to simply call `rake` than to call some command
of the testing framework that the developer has chosen. And if `rake` doesn't
run tests (it should, even if its default action is to build), `rake -T` isn't
too great a mental strain. Maybe that's just me, though.
More specs (IO-capture, Rainbow fixes)

For some reason, Rainbow doesn't monkey-patch String anymore (if it ever did),
so those specs were failing. This commit also adds a capture_io helper to the
spec support, and tests the output of a couple of lines of code, increasing
the coverage.

Incidentally, this commit was pre-approved with a rake ghundle.