Allows reverse lookup from filename

My guard file is setup to run acceptance/<file>_spec.rb,
controllers/<file>_rspec.rb and routing/<file>_routing_spec.rb, when
<file> is saved.

Some of my controllers have routing specs, some doesn't. In case they
doesn't spin will raise an error, because it can't invoke `empty?` on
nil.

This patch compmacts the `files_to_load` list, before testing if it's
empty. This fixes the problem described above.