move to v1 of mergo library

[skatkov]

Problem

I noticed that mergo library moved to vanity URL dario.cat/mergo and now all projects that depend on earlier version require "replace" rule.

replace github.com/imdario/mergo => github.com/imdario/mergo v0.3.16

Not a great experience!

Solution

Let's move taskin to stable version of mergo library and avoid this workaround.

I checked examples and they still work after the update.

[acidjazz]

thanks! checking now

[skatkov]

@acidjazz if there are any issues - happy to resolve.