Remove PHP 7.4 from `allow_failures`

[carusogabriel]

PHP 7.4 will be released soon and we need to ensure that Doctrine is compatible with it.

All we need to do is to remove these following lines

annotations/.travis.yml

Lines 45 to 46 in 793b91a

	allow_failures:
	- php: 7.4snapshot

and verify that everything is working.

This is a very good issue for those who want to participate in https://hacktoberfest.digitalocean.com.

[alcaeus]

Compatibility with PHP 7.4 was introduced in 1.7.0, but this wasn't merged down to master because of issues with other PRs that haven't been backported to 1.7 in time.

I've got some time set aside to fix this later today and will check how to credit @sunspikes for their contribution here.

[alcaeus]

Fixed by merging up 1.8 into master. #286 was repurposed to fix travis-ci failures in the build.