Bump version to PHP5.6 add compatibility testing and test for 5.6-7.4 only

Author: dshanske

.travis.yml

@@ -1,13 +1,12 @@
 dist: trusty
 language: php
 php:
-  - 5.4
-  - 5.5
   - 5.6
   - 7.0
   - 7.1
   - 7.2
   - 7.3
+  - 7.4
   - nightly
 env:
   - COMPOSER_REQUIRE=""

composer.json

@@ -11,13 +11,22 @@
     ],
     "bin": ["bin/fetch-mf2", "bin/parse-mf2"],
     "require": {
-        "php": ">=5.4.0"
+        "php": ">=5.6.0"
     },
     "require-dev": {
         "phpunit/phpunit": "4.8.*",
         "phpdocumentor/phpdocumentor": "v2.8.4",
-        "mf2/tests": "@dev"
+        "mf2/tests": "@dev",
+	"squizlabs/php_codesniffer": "^3.5",
+        "dealerdirect/phpcodesniffer-composer-installer": "^0.6",
+	"phpcompatibility/php-compatibility": "^9.3"
     },
+    "scripts": {
+	"install-codestandards": [
+		"Dealerdirect\\Composer\\Plugin\\Installers\\PHPCodeSniffer\\Plugin::run"
+	],
+	"phpcs": "./vendor/bin/phpcs -p"
+   },
     "autoload": {
         "files": ["Mf2/Parser.php"]
     },