Add support for php7.1 and up.

This is to add return type and type hinting support.
Everything above is optional. This way we can make sure,
we only get the correct types without the need to check them.

Also drop symfony/console 2 support for dev requirements.

.travis.yml

@@ -1,6 +1,7 @@
 language: php
 
 php:
+  - 7.1
   - 7.2
   - 7.3
 

composer.json

@@ -2,7 +2,7 @@
     "name": "php-deal/framework",
     "description": "Design by Contract framework for PHP",
     "require": {
-        "php": "^7.2",
+        "php": "~7.1",
         "goaop/framework": "^2.3"
     },
     "require-dev": {
@@ -12,7 +12,7 @@
         "roave/security-advisories": "dev-master",
         "slevomat/coding-standard": "^4.8",
         "squizlabs/php_codesniffer": "^3.4",
-        "symfony/console": "~2.7|~3.0|~4.0"
+        "symfony/console": "~3.0|~4.0"
     },
     "license": "MIT",
     "authors": [