Incompatibility with ruamel >= 0.18

[scarlehoff]

https://sourceforge.net/p/ruamel-yaml/code/ci/0.18.0/tree/CHANGES

Since 0.18.0 many functions that were used from ruamel are deprecated and at some point they've become errors. It might be possible to go up all the way to 0.18.2 but for the time being 0.18 is a safe cut.

At some point, if nobody deals with this and #59 I'll try to fix all problems and create a release, but I don't think I'll have time in the next few weeks or moths.