Switch to Ubuntu 18.04 for gitian building

For 0.17 we should aim to use Ubuntu 18.04 for building. This will be the new LTS release of Ubuntu.
We skipped 16.04 because of problems with the windows binary (#8732) but this was solved in 16.10, so ruling out new issues, the way should be clear.

This means finally building with a new C++ compiler, binutils, etc. Also this will solve a known security issue with the NSIS windows installer.