Add Laravel 5.8 support

[antonkomarev]

Closes #312

This PR adds:

• Laravel 5.8 support
• Allows to use Carbon 2.0 as well as Carbon 1.0

Additionally it sets minimum version of PHP 7.1.3 as all new Laravel versions has, because Laravel 5.8 MUST have methods with void return type and it's allowed only since PHP 7.1.

All versions below Laravel 5.5 cannot be supported because Illuminate\Support\Carbon has appeared only since v5.5.

[antonkomarev]

@cmgmyr check this out! 😉

[antonkomarev]

From Travis CI I've removed sudo: false and PHP versions below 7.1 and added PHP 7.3.

Do we need PHP nightly in build? It's failing all the time.

[odinns]

Thank you! :-) hoping for a quick merge.

[odinns]

@antonkomarev in your estimation, what is the likelyhood that this will be merged anytime soon?

I am considering to fork it and merge your PR from here :-)

[antonkomarev]

@oad-dwarf I've pinged Chris on Twitter. Hope it will be merged soon.

[cmgmyr]

Thanks for the work @antonkomarev! This has been tagged 2.17

[antonkomarev]

It seems that I've missed one thing, that for Immutable dates we will need to use new Date facade, but these changes will require to drop all the Laravel versions before 5.8.
I'm not sure that we need to do it right now.

[cmgmyr]

I'd like to hold off on that for a while. I think we should actively support the last 2-3 latest Laravel releases.

[antonkomarev]

@cmgmyr yeah, if anybody will really need to have Immutable Dates - he always can overwrite default model.