Update Virtualbox Mac installation instructions

Since macOS 10.13, Virtualbox installation procedure for macs may be slightly less straightforward as the installer will fail without giving the explanation for why this was the case (tried versions 5.1.38 and 6.1.12). As per https://forums.virtualbox.org/viewtopic.php?f=8&t=84092, the installer fails to direct the user to allow Oracle to make changes to the computer. Following the instructions allowed me to install Virtualbox version 6.1.12. ~~I am not sure if 5.1.38 is also installable but likely should be~~. However this overcoming this installation difficulty should be addressed in the installation manual on brightspace.

EDIT: I have been unable to install 5.1.38 to macOS 10.15.6. Therefore when updating the instructions we have to notify students to install a newer version of Virtualbox.

Installation instruction update:

Tell students that if running macOS 10.13+ they should install a newer version of Virtualbox.
Tell students that if running macOS 10.13+ the installation procedure involves going into Preferences > Security & Privacy > General > Allow Oracle to make changes.

Edited Aug 18, 2020 by Norman

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