Added compatibility with Symfony 4 (#12)

* Added compatibility with Symfony 4
* Improved travis config using more PHP versions

Author: marcortola

.travis.yml

@@ -1,5 +1,18 @@
 language: php
-php:
-  - '7.0'
-  - '7.1'
-before_script: composer install
+
+matrix:
+    include:
+        - php: 5.6
+        - php: 7.0
+        - php: 7.1
+    fast_finish: true
+
+before_install:
+  - composer self-update
+
+install:
+  - composer update --no-interaction
+
+script:
+  - composer validate --strict --no-check-lock
+  - phpunit

composer.json

@@ -16,7 +16,7 @@
         }
     ],
     "require": {
-        "symfony/console": "^3.2"
+        "symfony/console": "^3.2 || ^4.0"
     },
     "autoload": {
         "psr-4": {