Use the 'source' key's 'derivative' value rather than the 30s cutoff.

Excluding all tracks that are 30 seconds long is a bit questionable :) the file data has a field for this, however, since the sample files are derived files.
Merge pull request #2 from ianmcorvidae/patch-2

Use the 'source' key's 'derivative' value rather than the 30s cutoff.