Fix regression in rake task pattern handling.

Since our change to forward `task.pattern` to `rspec`
in #1653, it has not handled patterns that are relative
to the current directory for a side directory (e.g.
not under `spec`) of specs. This restores that behavior.

Fixes #1728.
Merge pull request #1734 from rspec/address-1715

Fix regression in rake task pattern handling.