Fix #188 - preserve timestamps format when converting from YAML to JSON

[m-radzikowski]

Fixes #188

SnakeYAML tries to be clever and detect data types, converting timestamps to Date. Then it's serialized to JSON in another date format.

This fix overrides the SnakeYAML config with a custom Resolver that does not detect timestamps and leaves them as a string.

[krasa]

Awesome, thanks!