Fix #81 (update snakeyaml version to 1.21)

[yborovikov]

This is a (minimalistic) fix for the backwards-incompatible refactoring in snakeyaml 1.20.

Note: if backwards compatibility with pre-1.20 snakeyaml is strongly desired we could introduce a wrapper with runtime detection of *Event constructor signatures. @cowtowncoder?

[cowtowncoder]

I think ideally I would avoid dynamic discovery/linkage if possible. But it all comes down to how common there are to have transitive dependencies. Also, unfortunately use of shading of snakeyaml is apparently problematic for OSGi (for reasons I don't even fully understand myself), solution which could otherwise remove problem altogether.

I think that one possible path here is to:

1. Keep Jackson 2.x at earlier version (pre-performance issue)
2. Make Jackson 3.0 require 1.20 or later (wrt compatibility)

jackson 3 is still quite a ways off (first release perhaps by end of 2018) so to me that seems plausible path.

[yborovikov]

Ok, in this case, we likely should:

1. Downgrade snakeyaml version back to 1.17 (re (yaml) Possible performance regression in snakeyaml 1.18 #67, especially if there is a reproducible benchmark/test and the degradation is measurable/significant) in Jackson 2.9.6?
2. Apply this PR to Jackson 3.0 master (and not the 2.9 branch).

Is my understanding correct?

[cowtowncoder]

@yborovikov Yes.

[asomov]

It should be better to use SnakeYAML engine in Jackson 3.0

[yborovikov]

Closing since the issue is fixed in snakeyaml 1.23.

[asomov]

@yborovikov : Yegor, I would like to make a pull request using the latest SnakeYAML release 1.23 and removing the deprecated methods.

Which branch should I use for this exercise ?

[yborovikov]

@asomov i'd probably put it in 2.10 - but i'm not the official maintainer.

[asomov]

Created #101