Add link to 7.4 / 7.4-buster now it's released

Author: stronk7

README.md

@@ -6,6 +6,7 @@ A Moodle PHP environment configured for Moodle development based on [Official PH
 
 | PHP Version  | Variant | Tags             | Status | Notes |
 |--------------|---------|------------------|--------|-------|
+| PHP 7.4      | Buster  | 7.4, 7.4-buster  | [![Build Status](https://travis-ci.org/moodlehq/moodle-php-apache.svg?branch=7.4-buster)](https://travis-ci.org/moodlehq/moodle-php-apache)|
 | PHP 7.3      | Buster  | 7.3, 7.3-buster  | [![Build Status](https://travis-ci.org/moodlehq/moodle-php-apache.svg?branch=7.3-buster)](https://travis-ci.org/moodlehq/moodle-php-apache)|
 | PHP 7.2      | Buster  | 7.2, 7.2-buster  | [![Build Status](https://travis-ci.org/moodlehq/moodle-php-apache.svg?branch=7.2-buster)](https://travis-ci.org/moodlehq/moodle-php-apache)|
 | PHP 7.1      | Buster  | 7.1, 7.1-buster  | [![Build Status](https://travis-ci.org/moodlehq/moodle-php-apache.svg?branch=7.1-buster)](https://travis-ci.org/moodlehq/moodle-php-apache)|