Add Cmake version to compilation instructions

[evd0kim]

Dear BlockSci developers!

I wish you could change section "Compiling BlockSci" and add Cmake version.

I'm not sure what version have to be to successfully compile the project, however, the pip3 install bug disappeared when I installed Cmake 3.11 and it hasn't installed modules when it was 3.5 (default Ubuntu Cmake version).

Thank you very much for your work.

[geofurb]

For Debian/Ubuntu, it's probably also helpful to add the lines:
sudo apt-get install gcc-7 g++7
and
update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 60 --slave /usr/bin/g++ g++ /usr/bin/g++-7

[nelsontodd]

These comments are great. I don't know if this is relevant here exactly, but over the summer me and a coworker made an ansible role that installs blocksci https://github.com/unchained-capital/blocksci-ansible-role

[evd0kim]

Debian 9. Non-default cmake version 3.13.3 is fine for BlockSci.