Restored support for PHP 5.5 and HHVM 3

[GrahamCampbell]

This PR restores support for PHP 5.5 and HHVM 3, to match Guzzle 6.5's supported versions. I have tested against the same version of HHVM that Guzzle 6.5 tests against. After this, I think we are good to tag v1.4.0.

NB The hacks I added in this PR can be undone in v2.0.0, which will certainly drop HHVM and PHP less than 7.2.

[Tobion]

After this, I think we are good to tag v1.4.0.

I think this should be v1.3.2 as there aren't any real features added since v1.3.1. In v1.4.0 we can then drop support for old php versions.

[GrahamCampbell]

1.3.2 is fine by me. We just need to change the branch alias from 1.3 to 1.4.

[Tobion]

Are you gonna tag v1.3.2?

[GrahamCampbell]

Before the 1.3.2 release, the change log also needs updating.

[GrahamCampbell]

In v1.4.0 we can then drop support for old php versions.

I don't think that's a good idea because:

1. Version 2.0 probably isn't that far off (maybe September/October with Guzzle 8.0)?
2. HHVM (non-)support isn't indicated in the composer.json file.
3. There is no debt in the src directory due to keeping PHP 5.5 and HHVM (especially since HHVM support is only partial), so there is actually no proper motivation to drop the versions (there will be motivation in 2.0, because breaking refactoring can occur and new features from PHP 7.2+ can be used, like scalar types).

[Tobion]

What's your plan with v2 of promises? What's the breaking change you plan?

[GrahamCampbell]

What's the breaking change you plan?

Adding scalar type hints, and removing the functions file, like in Guzzle, because global functions don't play nice when a different version of the package is installed globally with composer to locally, and leads to bugs like guzzle/guzzle#2511. See guzzle/guzzle#2546. The plan would be to add the new static class methods in 1.4.0 and deprecate the functions, then remove in 2.0.0. Some work already started at #108.