Change capability for EDD Add Ons and System Info sub menus

The "install_plugins" capability gives only WP super admins on WP multisite installs access to Add Ons, System Info Upgrade screen.
This is how it should be.
On single WP installs the Admin still can access these sub menus.
see
http://codex.wordpress.org/Roles_and_Capabilities
Merge pull request #1291 from robaggior/patch-2

Change capability for EDD Add Ons and System Info sub menus