In a new application I was adding in Slack notifications and while testing I got cURL error 60: SSL certificate problem: unable to get local issuer certificate...
I am working on a Windows 10 machine with xampp. I found the solution on Laracast Forum with a little bit of effort so I thought I would outline the steps succinctly to save a little time in the future.
I am going to assume that you have a version of Guzzle which is 6.2 or around that.
Step 1.
Download a fresh PEM file from this link https://gist.github.com/VersatilityWerks/5719158/download
This is a zip file which you will save into xampp.
Step 2
Unzip this file and save the file cacert.pem in D:/xampp/php/extras/ssl.
Step 3
Update the php.ini file. D:/xampp/php/php.ini
Alternatively to locate the php.ini file open the XAMPP control panel and click on Config adjacent to Apache. 
Search and find curl.cainfo and add the full path to the the cacert.pem file you just added.
The new line should be something like this:
curl.cainfo = D:\xampp\php\extras\ssl\cacert.pem
Step 4
Restart Apache and try and fire off your notification again. It should just work now.