Changes

Fix `--memory` attribute on Linux & other stuff
Changes:
* Fixed memory assignment otherwise, Vagrant would be assigning 
VirtualBox memory in giga bytes, not mega bytes:
```
==> default: Running 'pre-boot' VM customizations... A customization
command failed:
["modifyvm", :id, "--memory", 2]
```
* Use long form of `rsync` option `-z`.
* Fix vCPU assignment on Windows.
* Added requirements for Windows (`ssh`, `rsync`) to documentation.
* Typos.
Tested on openSUSE Leap 42.2 and Windows 10 Pro with latest Vagrant and 
VirtualBox.
 (commit: 75c5533)