Add more CIs (Rails 5.2, Webpacker v4 pre)

[ttanimichi]

• Against Rails 5.2 and Webpacker v3
• Against Webpacker v4 pre (allow_failures)

[zucler]

Hi there, has there been any progress on this? It would be great if you guys could add Webpacker 4.x support. Thanks in advance!

[BookOfGreg]

@zucler Webpacker hasn't properly released it's v4 in line with Webpack's v4 so I wasn't in a rush to get this out. I'll update the version of Webpacker to 4.0.0.rc.2 and see if it runs anyway. Our test suite is a little sensitive so will have to see if it can update cleanly or not.
#934

[petevk]

Hey there, I'm not sure if this was blocked on a stable release of webpacker v4, but as of a few minutes ago it's been released.

[StanBright]

For the record, I just upgraded https://www.saashub.com to use webpacker 4 and it seems to work fine with react-rails (2.4.7) 🎉