Chore/update appraisals

[BookOfGreg]

Summary

Adds Rails 5.2 + Webpacker 3, 4.pre versions to appraisal, with Webpacker 4.pre set as allow fail.
Closes #918

[BookOfGreg]

@ttanimichi Appears to be some failing test unrelated to this appraisal change. Must say in general I'm finding Travis a little flakey at times but this one does seem like a real failure on Webpacker 3.

[BookOfGreg]

This grid in Travis is roughly what I'm aiming for:

Gemfiles\Rubies	2.6.2	2.5.3	2.3.8	jruby-9.1.9.0
rails_4.2_sprockets_4		x
rails_5_no_sprockets_webpacker_1_1	x
rails_5_no_sprockets_webpacker_1_x	x
rails_5_no_sprockets_webpacker_2	x
rails_5_no_sprockets_webpacker_3	x
rails_5.2_no_sprockets_webpacker_3	x
rails_5.2_no_sprockets_webpacker_4	x	x	x	Allow Fail
rails_4.2_sprockets_2		x
rails_4.2_sprockets_3		x
rails_5_no_sprockets	x
rails_5.1_sprockets_4	x
rails_5.2_sprockets_4	x	x	x	Allow Fail

[BookOfGreg]

Reverted as Ruby 2.6.2 can't seem to gem install webdrivers

Parsing documentation for childprocess-0.9.0
Before reporting this, could you check that the file you're documenting
has proper syntax:

  /Users/me/.rbenv/versions/2.6.2/bin/ruby -c lib/childprocess/tools/generator.rb

RDoc is not a full Ruby parser and will fail when fed invalid ruby programs.

The internal error was:

	(NoMethodError) undefined method `[]' for nil:NilClass

ERROR:  While executing gem ... (NoMethodError)
    undefined method `[]' for nil:NilClass

[0xdevalias]

With this merged now, does that mean webpacker 4.x is actually supported? If so.. should the README get updated to say so?

Ref: #983

[BookOfGreg]

@0xdevalias thanks for asking for clarity on this.

Webpacker 4.x is supported in that it didn't crash the tests. I haven't personally tried it yet as my time is limited.
If you vouch it currently works (or can report bugs specifically for Webpacker 4.x) then yes we can call it supported.