Add Ruby 2.5 and 2.6 to travis configuration

[translunar]

From #617 it came to our attention that our Travis configuration is a bit out-of-date.

#610 is slightly different — this is correcting the existing configuration, which still needs to be done. But this issue, here, is just about adding 2.5 and 2.6 to the existing broken configuration.

[ngetahun]

Hey @MohawkJohn, is this issue still up for grabs? I'd like to make a PR.

[translunar]

Yes, please! The Travis configuration is a mess. Take a swing!

[translunar]

@ngetahun Did you end up having any luck?

[leesharma]

Hey there! It looks like this issue has been sitting for a few months—mind if I give it a go?

[translunar]

By all means!

…

On Mon, Oct 1, 2018 at 2:11 PM Lee Sharma ***@***.***> wrote: Hey there! It looks like this issue has been sitting for a few months—mind if I give it a go? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#619 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAKFpKnLevbEq1tZDZmFtD2lO7D0Sw5oks5ugoUIgaJpZM4TMqRI> .

[leesharma]

It looks like there's a compatibility issue between packable and ruby versions ≥ 2.5 (see SciRuby/packable#4). Because of this, tests that use their monkey-patched methods fail in CI for 2.5.1 and 2.6.0-preview2 (an example failure with the same error).

From the discussion on that issue, it looks like the problem isn't going to be resolved anytime soon. Luckily, the tests pass locally for me on OS X and Ubuntu; it looks like the problem is limited to CI and Debian systems. I think your options are:

• Switch to a different library for packing/unpacking binary data,
• Disable the broken functionality/tests on incompatible versions/systems, or
• Delay this issue until they release a patched version.

How do you want to handle this?

[translunar]

I'd say mark as pending on affected systems. It's difficult to extricate packable, but it's really only used in the Matlab IO, which should eventually become a plugin.