My Heroku deployment script installs LESS via NPM and is having trouble!

install_less.sh

#!/usr/bin/env bash
set -eo pipefail

if [ -x .heroku/python/bin/lessc ]; then exit; fi

npm install -g less

log.txt

-----> Running install_nodejs
-----> Running install_lessc
npm http GET https://registry.npmjs.org/less
npm ERR! Error: SSL Error: SELF_SIGNED_CERT_IN_CHAIN
npm ERR!     at ClientRequest.<anonymous> (/app/.heroku/vendor/node/lib/node_modules/npm/node_modules/request/main.js:525:26)
npm ERR!     at ClientRequest.g (events.js:193:14)
npm ERR!     at ClientRequest.EventEmitter.emit (events.js:93:17)
npm ERR!     at HTTPParser.parserOnIncomingClient [as onIncoming] (http.js:1462:7)
npm ERR!     at HTTPParser.parserOnHeadersComplete [as onHeadersComplete] (http.js:111:23)
npm ERR!     at CleartextStream.socketOnData [as ondata] (http.js:1367:20)
npm ERR!     at CleartextStream.CryptoStream._push (tls.js:526:27)
npm ERR!     at SecurePair.cycle (tls.js:880:20)
npm ERR!     at EncryptedStream.CryptoStream.write (tls.js:267:13)
npm ERR!     at Socket.ondata (stream.js:38:26)
npm ERR! If you need help, you may report this log at:
npm ERR!     <http://github.com/isaacs/npm/issues>
npm ERR! or email it to:
npm ERR!     <npm-@googlegroups.com>

npm ERR! System Linux 3.8.11-ec2
npm ERR! command "/app/.heroku/python/bin/node" "/app/.heroku/python/bin/npm" "install" "-g" "less"
npm ERR! cwd /app
npm ERR! node -v v0.8.12
npm ERR! npm -v 1.1.63
npm ERR! code ESSL
npm ERR!
npm ERR! Additional logging details can be found in:
npm ERR!     /app/npm-debug.log
npm ERR! not ok code 0

I realized after posting this that part of the problem is that the test used in the script to avoid installing LESS if it is already present is wrong – it fails because the -x (executable) option does not return true for symbolic links to executables. Having fixed this the npm command is no longer being called & I can get on with deploying changes.