chore: update composer dependencies, php version and travis build

Author: Harrison Ifeanyichukwu

.travis.yml

@@ -1,8 +1,7 @@
 language: php
 php:
-  - 5.6
-  - 7.0
   - 7.1
+  - 7.2
 before_script:
   - composer self-update
   - composer install

composer.json

@@ -10,12 +10,12 @@
         }
     ],
     "require": {
-        "php" : ">=5.6",
-        "php-coveralls/php-coveralls": "^2.1"
+        "php" : ">=7.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^7.3",
-        "squizlabs/php_codesniffer": "^3.3"
+        "squizlabs/php_codesniffer": "^3.3",
+        "php-coveralls/php-coveralls": "^2.1"
     },
     "autoload": {
         "psr-4": {