Update PHP version support

Changed:
- Follow Composer's support for PHP 5.3+.
- Move php-coveralls from composer.json to .travis.yml

.travis.yml

@@ -1,17 +1,29 @@
-sudo: false
-
+os: linux
+dist: bionic
 language: php
-php:
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
-  - nightly
 
 matrix:
+  include:
+    - php: 5.3
+      dist: precise
+    - php: 5.4
+      dist: trusty
+    - php: 5.5
+      dist: trusty
+    - php: 5.6
+      dist: trusty
+    - php: 7.0
+      dist: xenial
+    - php: 7.1
+    - php: 7.2
+    - php: 7.3
+    - php: 7.4
+      env:
+        - deps=high
+        - SYMFONY_PHPUNIT_VERSION=7.5
+    - php: nightly
   fast_finish: true
   allow_failures:
-    - php: 5.6
     - php: nightly
 
 branches:
@@ -20,14 +32,22 @@ branches:
 
 cache:
   directories:
-    - vendor
+    - $HOME/.composer/cache
+
+before_install:
+  - composer self-update
 
-before_script:
-  - mkdir -p build/logs
-  - composer update --dev --no-interaction
+install:
+  # flags to pass to install
+  - flags="--ansi --prefer-dist --no-interaction --optimize-autoloader --no-suggest --no-progress"
+  # update deps to latest in case of high deps build
+  - if [[ "$deps" == "high" ]]; then composer config platform.php 7.2.4; composer update $flags; fi
+  # install dependencies using system provided composer binary
+  - composer install $flags
 
 script:
   - ./vendor/bin/simple-phpunit
 
 after_script:
-  - if [[ "$TRAVIS_PHP_VERSION" == '7.1' ]]; then php vendor/bin/php-coveralls -v ; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == '7.4' ]]; then composer require php-coveralls/php-coveralls ; fi
+  - if [[ "$TRAVIS_PHP_VERSION" == '7.4' ]]; then php vendor/bin/php-coveralls -v ; fi

composer.json

@@ -10,11 +10,11 @@
         }
     ],
     "require": {
+        "php": "^5.3.2 || ^7.0",
         "composer-plugin-api": "^1.0"
     },
     "require-dev": {
         "composer/composer": "^1.0",
-        "php-coveralls/php-coveralls": "^2.0",
         "symfony/phpunit-bridge": "^3.4"
     },
     "conflict": {