build: Add testing for PHP 7.4

* Fix the PHP 7.3 build, which recently started failing
  due to Travis upgrading php-xdebug to a version that no longer
  provides its coverage by default, causing PHPUnit's coverage job
  to get confused and emit an error.
  Ref xdebug/xdebug#699.

* Update php-coveralls to a version that supports and is tested with
  PHP 7.4 (this version also dropped HHVM support).
  <https://github.com/php-coveralls/php-coveralls/blob/v2.4.3/CHANGELOG.md>

Author: Krinkle

.travis.yml

@@ -1,15 +1,18 @@
-sudo: false
-dist: trusty
 language: php
 php:
+  - "7.4"
   - "7.3"
   - "7.2"
+env:
+  global:
+    # Since PHP 7.3, php-xdebug is disabled by default unless this is set.
+    - XDEBUG_MODE=coverage
 install:
   - composer install
 script:
   - composer test
 after_success:
-  - wget https://github.com/php-coveralls/php-coveralls/releases/download/v1.0.2/coveralls.phar
+  - wget https://github.com/php-coveralls/php-coveralls/releases/download/v2.4.3/coveralls.phar
   - chmod +x coveralls.phar
   - ./coveralls.phar -v
 notifications: