Fix jsyaml build by replacing istanbul with istanbul-classic

[ingydotnet]

No description provided.

[perlpunk]

Thanks.
I'm still getting a deprecation warning:

+ cd js-yaml-3.13.1
+ perl -pi -e 's/"istanbul": "\^0\.4\.5",/"istanbul-classic": "^2019.8.9",/' package.json
+ npm install .
npm WARN deprecated core-js@2.6.11: core-js@<3 is no longer maintained and not recommended for usage due to the number of issues. Please, upgrade your dependencies to the actual version of core-js@3.                                                      
npm WARN deprecated circular-json@0.3.3: CircularJSON is in maintenance only, flatted is its successor.

Maybe we should just hope that the js-yaml package is fixed by the maintainers soon...