Install dcm2niix in CI

[kousu]

Fixes #27

This upgrades the CI environment from Ubuntu 16.04 LTS (which is Very Old now) to 18.04 (which is Current Boring Mainline), because 16.04 is too old to have dcm2niix available, but 18.04 does and is probably closer to what most people are running anyway!

[kousu]

Tagging both Julien and Alex; but you can sort out which of you reviews this first.

[jcohenadad]

awesome! much more elegant approach than what i suggested. when we will be testing other OSs we might want to find an alternative solution

[kousu]

Yeah. Well. In my opinion conda counts as a separate OS, just like Java or docker does. Except there's always some lhttps://www.joelonsoftware.com/2002/11/11/the-law-of-leaky-abstractions/eakiness from the underlying platform, so I would rather just adapt to that platform directly than trying to paper over it. This is the approach advocated by Drew Devault.

That would means on macOS running brew install dcm2niix and on Windows choco install dcm2niix or using curl'ing the prebuilt binaries for Windows. Those are the most canonical ways to install out-of-language dependencies for those platforms. In broader context, eventually, someone could package shimming-toolbox into brew or apt and add the dependency in their brew/apt build script; if we used conda in our code then packagers for those platforms would need to do surgery to remove it and fit the package in to their system. It's not impossible, but it's somewhat unfriendly.