Add PHP 7.4 to Travis CI, remove PHP 5.5 from Travis CI

.travis.yml

@@ -4,14 +4,14 @@
 
 language: php
 
-# Travis is migrating to Xenial, which does not support PHP 5.5 so we need to
-# force use of Trusty until we raise our minimum PHP version requirement.
-dist: trusty
-
 # -----------------------------------------------------------------------------
 # Environment setup and test scripts execution
 #
 
+services:
+  - postgresql
+  - mysql
+
 before_install:
   # If PHP >= 7.0, force use of PHPunit 5.7
   - if php -r "exit( (int)! version_compare( '$TRAVIS_PHP_VERSION', '7.0', '>=' ) );"; then mkdir -p ~/bin && wget -O ~/bin/phpunit https://phar.phpunit.de/phpunit-5.7.phar && chmod +x ~/bin/phpunit; fi
@@ -27,12 +27,12 @@ script:
 #
 
 php:
-  - 5.5
   - 5.6
   - 7.0
   - 7.1
   - 7.2
   - 7.3
+  - 7.4
 
 env:
   - DB=mysql