Change the PHP support to version 7.2 or aboce

Since there will still be secutiry support until 30 Nov 2020 and there
was something happening in Travis when generating coverage with newer
versions of PHP which I'm not in the mood to debug, I decided to change
the support again, this time to version 7.2.

Signed-off-by: Henrique Moody <[email protected]>

Author: henriquemoody

.travis.yml

@@ -2,6 +2,7 @@ sudo: false
 language: php
 
 php:
+  - 7.2
   - 7.3
   - 7.4
   - nightly

composer.json

@@ -15,7 +15,7 @@
         "sort-packages": true
     },
     "require": {
-        "php": ">=7.3",
+        "php": ">=7.2",
         "respect/stringifier": "^0.2.0",
         "symfony/polyfill-mbstring": "^1.2"
     },

docs/installation.md

@@ -7,4 +7,4 @@ you can install it using [Composer](http://getcomposer.org).
 composer require respect/validation
 ```
 
-Works on PHP 7.3 or above.
+Works on PHP 7.2 or above.