Add support for silverstripe 5

[satrun77]

Update composer to support silverstripe 5. Tested with CMS 5 beta-1

[UndefinedOffset]

Part of me wonders given the APIs are so similar if this should be something like, we could tighten it down later if the APIs diverge to the point that it causes issues.

• silverstripe/framework ^4 | ^5
• silverstripe/vendor-plugin ^1.0 | ^2.0
• silverstripe/spamprotection ^3 | ^4

[UndefinedOffset]

Have a look at https://github.com/webbuilders-group/silverstripe-turnstile/blob/master/composer.json, also I'm pretty sure we can drop silverstripe/vendor-plugin now it's part of the core's dependencies so it's not needed even in 4.x.

[satrun77]

@UndefinedOffset PR updated

[UndefinedOffset]

Thanks :)