Add support for PHP 7.4, drop support for 7.1 (#61)

* Update composer.json for PHP 7.4

* Drop support for PHP 7.1, add support for 7.3 and 7.4

Co-authored-by: Nicholas Law <nick@nicholaslaw.com.au>

.gitignore

@@ -5,3 +5,4 @@ vendor/
 .idea
 .DS_STORE
 .php_cs.cache
+.phpunit.result.cache

.travis.yml

@@ -1,7 +1,8 @@
 language: php
 php:
-  - '7.1'
   - '7.2'
+  - '7.3'
+  - '7.4'
 
 before_script:
   - composer self-update

composer.json

@@ -16,14 +16,14 @@
         }
     ],
     "require": {
-        "php": ">=7.1.0",
+        "php": ">=7.2.0",
         "ext-json": "*",
         "guzzlehttp/guzzle": "~6.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.13",
-        "phpspec/phpspec": "^5.1",
-        "phpunit/phpunit": "^5.7"
+        "phpspec/phpspec": "^6.1",
+        "phpunit/phpunit": "^8.5"
     },
     "autoload": {
         "psr-4": {