Bug/3278 - Unrequire cu_slider module

[zrhoffman]

Fixes #3278. The cu_slider module is required in its .info file, so on new sites, it is enabled before any of its dependencies, particularly libraries.

This is a problem for install.php, because after cu_slider is enabled, cu_slider_init() runs on each XHR to update the status bar and start another job in the "Install profile" step. Since cu_slider_init() always calls libraries_get_path(), an error will occur.

If we unrequire cu_slider, it will be enabled after libraries is enabled, and the "Install profile" step of install.php is able to complete. Because cu_slider is a dependency of Express itself, it will always stay enabled, despite it not being required.

This pull request also makes libraries an explicit dependency of cu_slider.