[seL4] Camkes manifest now requires cmake 3.7.2 or higher to be built

I was running the following commands to build camkes:

1. repo init -u https://github.ls com/sel4/camkes-manifest.git 2. repo sync 3. make clean 4. make ia32_simple_defconfig; make silentoldconfig 5. make -j4 6. qemu-system-i386 -nographic -m 512 -cpu Haswell -kernel images/kernel-ia32-pc99 -initrd images/capdl-loader-experimental-image-ia32-pc99

Earlier it used to work perfectly fine but now it requires cmake 3.7.2 or higher which can be obviously be done by using pip install.

I just wanted to convey this so that the same may be updated on prerequisites page. (https://github.com/SEL4PROJ/sel4-tutorials/blob/master/Prerequisites.md)