Bump version from 7.3 to 8.1

[ksylvest]

Also freezing version constant and switching from double to single quotes to match style used in project.

[runlevel5]

LGTM 👍

[runlevel5]

@ksylvest Would you be able to verify if this new version works fine with https://github.com/rubyjs/mini_racer?

[ksylvest]

@runlevel5 Thanks for the response / review. I ran into some issues during that verification:

#298

Did you want to hold off on the merge until that is fixed?

[runlevel5]

@ksylvest I would hold off any version bump till mini_racer catches up.

The recent bump to latest 7.3 still causes issue with mini_racer

[nightpool]

Thanks for your contribution! After many months and many contributors, we finally managed to upgrade to V8 8.4 here: #301, with the corresponding mini_racer PR here: rubyjs/mini_racer#166

Thanks again!