A recent change has been made in NPM’s infrastructure in Heroku. Last month, a change to npm’s infrastructure was recently made that caused many users to receive a SELF_SIGNED_CERT_IN_CHAIN error during installation and publishing of packages. The npm-recommended fix is to set a blank value for ca in your npm config. If you’ve encountered this error before, it should now be fixed.
This issue can be circumvented by upgrading to node 0.10.26 or greater, but the upgrade is not required. But if ever you upgraded, then you must have encountered this error. The node buildpack has been updated to automatically generate an .npmrc for your app with the blank ca entry, if one is not already present.
Anyway, it’s already fixed. You can check out more updates at the Heroku devcenter changelog page.