The Medialibrary package requires PHP 5.5.9+ and Laravel 5.1.0+. To create derived images GD needs to be installed on your server. If you want to create PDF thumbnails Imagick is also required.

GD and Imagick can be installed using apt-get on Ubuntu and Debian:

$ apt-get install php5-gd imagemagick php5-imagick

Or with yum on CentOS:

$ yum install php55-gd ImageMagick ImageMagick-devel

Note: Root access to your server is probably required.