Fatal error: Class 'NexmoMessage' not found

[Ethan Pransky]

Not sure why I'm getting this. I'm not a pro with composer, but I included this package like so:

"require" : {
"prawnsalad/nexmo": "dev-master"
}

The package gets built in my vendor folder no problem, but when I try running:

require 'vendor/autoload.php';
$sms = new NexmoMessage(NEXMO_KEY, NEXMO_SECRET);
$sms->sendText($phone, "app", "yo");

I get the error: Fatal error: Class 'NexmoMessage' not found. Any idea why this could be? I also tried explicitly use NexmoMessage, but that didn't work either. Thanks