I am using OS X Yosemite
I ran the following command in Composer because Laravel fails to download and install properly all the time:
composer diagnose
Another possible reason is that the server's SSL cert has expired. If it is your server, check the /etc/apache2/ssl (for ubuntu). The certs usually last for 365 days. See 'https://www.digitalocean.com/community/tutorials/how-to-create-a-ssl-certificate-on-apache-for-ubuntu-14-04' on how to create one.