Auto bump k6 version and run with multiple versions

[mstoykov]

From #119 it became apparent we do not bump k6 version regularly.

It also is probably good idea to not bump it to the latest one but instead test all the libraries with the last X number of k6 versions. Maybe even with the current master.

It will be nice if those are easily configurable and we can do them per library.

As it might turn out we add a jslib that uses a brand new functionality.

On the other hand this repo is mostly to store versions not to actually develop jslibs so maybe all of this should be moved to a template repo and used by all the different jslib repos instead of having it here.