# prepare
$ sudo apt-get install -y linux-headers-generic build-essential dkms
$ sudo apt-get -y purge virtualbox-guest*

# get the right ISO from http://download.virtualbox.org/virtualbox/
$ wget http://download.virtualbox.org/virtualbox/5.0.16/VBoxGuestAdditions_5.0.16.iso

# mount the ISO
$ sudo mount -o loop,ro VBoxGuestAdditions_5.0.16.iso /mnt

# install the guest additions
$ sudo sh /mnt/VBoxLinuxAdditions.run

# unmount the ISO
$ sudo umount /mnt

# remove the ISO
$ rm VBoxGuestAdditions_5.0.16.iso

One-Liner

VERSION='5.0.16' && \
sudo apt-get install -y linux-headers-generic build-essential dkms && \
sudo wget "http://download.virtualbox.org/virtualbox/$VERSION/VBoxGuestAdditions_$VERSION.iso" && \
sudo mkdir /media/VBoxGuestAdditions && \
sudo mount -o loop,ro "VBoxGuestAdditions_$VERSION.iso" /media/VBoxGuestAdditions && \
sudo sh /media/VBoxGuestAdditions/VBoxLinuxAdditions.run && \
sudo umount /media/VBoxGuestAdditions && \
sudo rm -rf "VBoxGuestAdditions_$VERSION.iso" && \
sudo rmdir /media/VBoxGuestAdditions