Fix duplicate symbol error when linking

When running specific specs, it was possible for a single spec to be compiled twice, and then linked. The linker would choke on this, claiming a duplicate symbol. This pull fixes that bug by expanding the filename for each path before passing it to the rake spec task.

This was triggered by an incorrect `Guardfile` template included with `guard-motion`. The template was updated, as well as the README. The latter two fixes are not strictly necessary, but are included for completeness.