Error installing kernel from source!

Question

I compiled the latest Linux Kernel (4.5.x) from Linus's git tree. The Kernel compiled perfectly with no errors. After compiling the kernel I installed the modules using sudo make modules_install. That worked flawlessly too; however when I run sudo make install it gives me this error.

 sh ./arch/x86/boot/install.sh 4.5.0-00001-gf99c49a arch/x86/boot/bzImage \
System.map "/boot"
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 4.5.0-00001-gf99c49a /boot/vmlinuz-4.5.0-00001-gf99c49a
run-parts: executing /etc/kernel/postinst.d/dkms 4.5.0-00001-gf99c49a /boot/vmlinuz-4.5.0-00001-gf99c49a
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 4.5.0-00001-gf99c49a /boot/vmlinuz-4.5.0-00001-gf99c49a
update-initramfs: Generating /boot/initrd.img-4.5.0-00001-gf99c49a
W: Possible missing firmware /lib/firmware/i915/skl_guc_ver4.bin for module i915

gzip: stdout: No space left on device
E: mkinitramfs failure cpio 141 gzip 1
update-initramfs: failed for /boot/initrd.img-4.5.0-00001-gf99c49a with 1.
run-parts: /etc/kernel/postinst.d/initramfs-tools exited with return code 1
arch/x86/boot/Makefile:189: recipe for target 'install' failed
make[1]: *** [install] Error 1
arch/x86/Makefile:260: recipe for target 'install' failed
make: *** [install] Error 2

I followed these three links: 1, 2 and 3; but they don't solve the problem.

I think space (in /boot) is the issue as sometimes I get a warning which says that there is only 24 MB space left in /boot however df shows :-

Filesystem     1K-blocks      Used Available Use% Mounted on
udev             1833688         0   1833688   0% /dev
tmpfs             395944     23612    372332   6% /run
/dev/dm-0      475786312 234152100 217442632  52% /
tmpfs            1979716      1052   1978664   1% /dev/shm
tmpfs               5120         4      5116   1% /run/lock
tmpfs            1979716         0   1979716   0% /sys/fs/cgroup
/dev/sda2         650875    398220    219070  65% /boot
/dev/sda1         102178      7236     94942   8% /boot/efi
tmpfs             395944        60    395884   1% /run/user/1000

NOTE: i am currently running Linux Kernel 4.5.0-rc1 and this error only shows up when I compile the kernel with make oldconfig. If I use make defconfig then everything works fine.

EDIT 1: Ubuntu 15.10