repeatscout 1.0.5: Add optional dependency on trf

To filter your repeat library to remove low-complexity and tandem sequences,
you will neeed to have perl 5.5 (or better; see http://www.perl.com), nseg
(Wooton and Federhen, 1993; see ftp://ftp.ncbi.nih.gov/pub/seg/nseg) and trf
(Benson, 1999; see http://tandem.bu.edu/trf/trf.html).

See http://bix.ucsd.edu/repeatscout/README.txt
See #1338