pubsubhubbub php-subscriber Error

I am trying to install pubsubhubbub/php-subscriber in laravel 5.1. While installing, I am facing a problem listed below.

Your requirements could not be resolved to an installable set of packages.

Problem 1

• The requested package pubsubhubub/subscriber could not be found in any version, there may be a typo in the package name.

Potential causes:

• A typo in the package name
• The package is not available in a stable-enough version according to your minimum-stability setting see http://ift.tt/1klt5D3> for more details. Read http://ift.tt/1irpAoX for further common problems.

I have no idea how to resolve this issue??

via Chebli Mohamed