Upgrade to libv8 8.4

[nightpool]

Based on #163 with a slight tweak to the gemspec and tests. Requires rubyjs/libv8#301 to be merged.

Future versions of v8 (8.4 stable, 8.5 beta, and 8.6 development) are blocked on https://bugs.chromium.org/p/v8/issues/detail?id=10041. Right now you can test locally by installing https://github.com/nightpool/libv8/packages/314620, but be aware that I've only pushed a version for x86_64-linux, since that's what I was testing on (specifically, i tested on the ruby:2.5.8-stretch docker container).

rake test passes locally, I haven't tried any of the other tests yet.

closes #153, #163

[SamSaffron]

This is fantastic @nightpool ... @ignisf can we merge this?

[nightpool]

Unfortunately i'm still having problems with my React SSR related to locale data. I'm working on a further patch for libv8, but my current working theory is that libv8 isn't packaging locale data at all. Adding it statically to the v8 so looks pretty easy, but maybe it should do so as a separate gem or an optional configuration?

[SamSaffron]

I would say it depends on how much extra size would be added by adding locale data?

[nightpool]

Okay, it looks like the locale data isn't too much bigger—my libv8 .gem went from 11.9 MB to 16.2 MB.

Not as bad as I was expecting, but still a pretty big increase.

[SamSaffron]

Will leave this for @ignisf to decide but my gut feel is that it is OK.

Is there are way to package it so it is an additional dependency eg: libv8-locale ?

[nightpool]

I haven't looked into it yet. I'm sure it's possible, not sure what the additional complexity would be or how long it would take to get working.

[nightpool]

I'm going to push two more PRs today to trigger travis tests with my pre-release libv8 packages, just so we can make sure tests are working on the full suite of platforms as well as locally for me.

Wait this isn't going to work because i don't have darwin binaries. Oh well.

[SamSaffron]

@nightpool shall I merge this now? is is backwards compat?

[nightpool]

@SamSaffron I haven't investigated the c++ changes in detail, because I was mostly building on the work @cataphract and @ignisf did, but my guess is probably not. (especially since I bumped the gemfile to require libv8 8.4, which hasn't been released yet)

[SamSaffron]

K ... going to hold off on merging this till we get libv8 8.4 published.

[nightpool]

@SamSaffron @cowboyd @ignisf built libv8 gems are ready at https://github.com/rubyjs/libv8/releases/tag/v8.4.255.0, I've tested them locally and we're just waiting on getting them pushed to rubygems before we can merge this!

[SamSaffron]

Amazing work @nightpool I just pushed a release to rubygems! merging this in!

[nightpool]

yes!!!!!!!! so great to get this merged.