Bug:

$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
.....
Setting up linux-image-4.2.0-34-generic (4.2.0-34.39) ...
Running depmod.
update-initramfs: deferring update (hook will be called later)
Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 4.2.0-34-generic /boot/vmlinuz-4.2.0-34-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 4.2.0-34-generic /boot/vmlinuz-4.2.0-34-generic
update-initramfs: Generating /boot/initrd.img-4.2.0-34-generic
Warning: /sbin/fsck.btrfs doesn't exist, can't install to initramfs, ignoring.

Solution:

$ sudo ln -s /bin/fsck.btrfs /sbin/
$ sudo update-initramfs -u
update-initramfs: Generating /boot/initrd.img-4.2.0-34-generic