Use Process.spawn instead of the child_process gem

[eregon]

Since aruba is Ruby 2.6+ it can use Process.spawn which exists since 2.4.

Process.spawn covers the whole API of child_process, so there is no need for that dependency and it should be fairly easy to remove it.
child_process is unfortunately not very actively maintained, and that causes issues, for instance oracle/truffleruby#1525 / enkessler/childprocess#172.

🤔 What's the problem you're trying to solve?

Getting aruba and simplecov to work well under modern Rubies.

[mvz]

@eregon could you please have a look at #892? Since you implemented enkessler/childprocess#175 I'd like your input on this.