Drop HHVM support

[SimonFrings]

Due to HHVM's decrease in use, there's no more need to support it.

For reference take a look at the similar conversation in clue/confgen#37 or clue/confgen#39.

[clue]

@SimonFrings Thanks for looking into this!

I'm not going to argue there's a lot of value in supporting a legacy HHVM build that accounts for less than 0.1% of our installation base, but I would argue that at this point fixing legacy support is actually easier than dropping it entirely. See reactphp/socket#289 for example.

Dropping HHVM could be seen as a BC break (admittedly one that affects hardly anybody), so I'd rather play safe here. Starting a v3 just for HHVM seems excessive at this point, but I'm definitely open to getting this in once work on the v3 starts, see also #118 and others.

WDYT?

[SimonFrings]

@clue Guess you're not wrong with that.

This seems like a topic for a version 3.x but that's somewhere in the future. I will come back to this if there's an upcoming release for a new major version. 👍