CI: Set dist: trusty for Rubinius

[olleolleolle]

This PR

• configures the Travis "dist" to a Rubinius-supporting one.
• adds gems that rbx needs to the gemspec

When we know more which of these matrix combination elements are even theoretically supported, we may be able to make the matrix smaller.

[olleolleolle]

File does not exist: racc/parser.rb

Racc/parser.rb missing seems hard to do anything about. Update: aha, code not loaded, see next commit.

The depreciation warning from mocha is fixable but changes nothing for the CI situation for Rubinius. Update: #482 does that

Perhaps it's not possible to run Rubinius with mocha in the test suite.

[olleolleolle]

https://github.com/teamcapybara/capybara/pull/1194/files#r7912212 This comment in Capybara (2013) explains about the code loading.

I gained more clues in a Bundler issue:
rubygems/bundler#2774

[olleolleolle]

@kaspth Thanks for the confidence, but we're closer but not quite there yet.

The json extension can not be built completely, yet, on Rubinius.

Dear Rubinius team & community: I made a tiny PR trying to get to install json support. What's the best way for me to do that? #481
my current failure is: JSON extension fails on ST_CONTINUE not being set up. I learned that in the oniguruma build/ directory in the platform's repository, there was a st.h header file which defined it as part of an enum. https://travis-ci.org/github/rails/jbuilder/jobs/663411968#L1087 Travis link to my failure.
rubinius/rubinius#3795 (comment) I read this comment, and now I thought "perhaps I'm trying to do things that... won't work".

(I asked the above, in the Rubinius gitter channel. I'll try to keep on top of the issue if I get any answer.)

[kaspth]

Ah, gotcha. Figured the 5.2 fix would carry over here, in any case this is better than what was before even if not all the way there. Thanks for taking this on 🙏

[rafaelfranca]

Can we remove rubinius from our build matrix? It is not a "Ruby" anymore.