Update to require PHP 7.1+

[WyriHaximus]

This changeset updates the project to require PHP 7.1+ and drop legacy PHP < 7.1 and HHVM as discussed in #173. I'm marking this as a BC break for anybody still stuck on very old PHP versions, but there's little chance this will affect any current projects otherwise.

This PR aims to contain the minimal changeset to update the PHP version requirement only. Follow-up PRs will update our APIs to leverage newer language features.

Builds on top of #172, #174 and others
Refs reactphp/cache#58

[SimonFrings]

Thanks for working on this @WyriHaximus, I added a few remarks down below and I also went through the project and noticed a few things we should add to this PR. Here are my findings and thoughts:

1. First of all, I think my suggested changes will make this PR a lot bigger, so maybe it's a good idea to split the last commit into two. This way the PR is a bit more overseeable. Could be something like:

   • Commit 2: "Update PHP language syntax and remove legacy workarounds" (including all the array stuff)
   • Commit 3: "Update test suite and remove legacy PHPUnit workarounds" (PHPUnit changes + legacy loop extensions in tests)

2. We have a lot of workaround functions inside the TestCase.php that were necessary for PHPUnit < 7.5. Tried this out and I think we can get rid of all of them (regarding the setExpectedException(), assertContainsString(), assertContainsStringIgnoringCase() and assertSameIgnoringCase())

3. There are 3 places in our test suite where we skip tests for HHVM, we should also remove them (reagarding the DuplexResourceStreamTest, WritableResourceStreamTest and ReadableResourceStreamTest)

4. Inside the DuplexResourceStream and ReadableResourceStrem we have legacy PHP 5 workarounds at the bottom which can be removed in my opinion.

5. In line 90 inside the FunctionalInternetTest class is a comment describing a bug in PHP < 7.1.4 and PHP < 7.0.18. The part for 7.1.4 should stay, but I think we can remove the part for PHP < 7.0.18.

That's it for now 👍

[WyriHaximus]

Thanks for the pointers @SimonFrings, updated the PR according to them.

[SimonFrings]

Went through everything, looks good to me. Also nice that you caught a few additional things that I didn't even mention 👍

[WyriHaximus]

Update one small nitpick, and had to bump the macos job a few times until it passed

[clue]

@WyriHaximus Thank you for looking into this and for your patience! I've added some minor remarks only, otherwise love the direction!

[clue]

@WyriHaximus Went over your changes together with @SimonFrings and addressed all outstanding issues, great work, now let's get this shipped!

For reference: Link to diff between original 1d047d5 and new 2f70d54: https://gist.github.com/clue/7e6012fecc995dee201eb3aae600bcfb)

[SimonFrings]

Went over all changes made, looks good to me so let's get this in 👍