Explicitly check for rubinius before using fakefs.
Shift output to a test directory in specs.

Defaults to using FakeFS. But if it can't be used,
shift the output to a special test directory so that
we don't interfere with any historical data.
Fix problem with rspec double on ruby192.
Merge pull request #98 from robincurry/bug/97

Fix issues with specs stepping on historical data.