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

DoctrineMongoDBBundle/.travis.yml

Lines 16 to 17 in 227d908

	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]

Fixed in #588.