Drop support for PHP < 7.3

I am about to release version 2.0, and it's been way too much time since
I created a MINOR or MAJOR version.

Because I don't know when will be the next time I will release the next
version and it is troublesome to keep support to old PHP versions, I
decided to only give support to the versions PHP is actively supporting.

Signed-off-by: Henrique Moody <henriquemoody@gmail.com>

.travis.yml

@@ -2,8 +2,6 @@ sudo: false
 language: php
 
 php:
-  - 7.1
-  - 7.2
   - 7.3
   - 7.4
   - nightly

composer.json

@@ -15,7 +15,7 @@
         "sort-packages": true
     },
     "require": {
-        "php": ">=7.1",
+        "php": ">=7.3",
         "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 7.1 or above.
+Works on PHP 7.3 or above.