Test 5.4 and 5.5 just to be sure

[WyriHaximus]

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[clue]

Thanks @WyriHaximus, as always, your suggested PRs are very much appreciated 👍

Currently, we're only testing the oldest supported version and the latest release. Do you see a particular need to test intermediary versions?

I'm not saying we shouldn't merge this PR, but I'm a bit reluctant considering that we may end up with dozens of versions in the years to come :) I suppose this is not the first project to run into this, has any other project already achieved consensus?

[WyriHaximus]

In my honest opinion we should test all versions we support. With the recent 5.5.13 BC break debacle, I prefer to know (at least in my personal projects) when something goes wrong with a new version. (Now I don't know how often Travis updates PHP binaries tbh.) It's also a save guard that everything works in all versions a project supports, and for my own projects I prefer only to support the PHP versions not EOL yet. So I don't see a big rise in PHP versions in this file any time soon 😄.

[clue]

[…] support the PHP versions not EOL yet. So I don't see a big rise in PHP versions […]

Agreed. Despite that I personally still don't see a particular need for testing intermediary versions for this library, I also don't see a particular reason to not test them.

So why hold this back? :) Thanks for the contribution! 👍