Update mini_racer gem dependency

Why is this change being made?

As of 2021-11-09, The project can't seem to be built from scratch anymore (bundle install) on MacOS Monterrey (RUBYGEMS PLATFORM: x86_64-darwin-21) because there are no binaries yet for this platform, and there is an error attempting building the native libv8 gem.

This MR updates the mini_racer gem, which in turn updates the libv8 gem. This allowed me to build this project locally with bundle install.

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