Dependency obsolete causing compilation error at OTP 18

[DanielsLuz]

The dependency to plain_fsm (from gburd/plain_fsm) seems to be obsolete, the most recent repository is from uwiger/plain_fsm.

This current old one triggers another dependency to an obsolete edown (from esl/edown) which won't compile at OTP 18, the most recent is from uwiger/edown.

Uncommenting the edown (which is correct in this repository, but commented) dependency should do, but the plain_fsm, should be updated too I guess?.