Please change source for "Flexichain"

Concerning https://github.com/quicklisp/quicklisp-projects/blob/master/projects/flexichain/source.txt

Current definition, but not maintained
`https https://common-lisp.net/project/flexichain/download/flexichain_latest.tgz`
Should be changed to:
`git https://github.com/robert-strandh/Flexichain.git`

This version adds definitions for ecl and clasp (and years of fixes)