feat(): phpunit 7 and remove travis php5.6, php7.0

Author: al3xLvs

.travis.yml

@@ -1,8 +1,6 @@
 language: php
 
 php:
-  - 5.6
-  - 7.0
   - 7.1
   - 7.2
   - 7.3

composer.json

@@ -20,7 +20,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "5.7.*",
+        "phpunit/phpunit": "^7",
         "php-coveralls/php-coveralls": "2.1.0"
     },
     "scripts": {

phpunit.xml

@@ -1,13 +1,4 @@
-<phpunit
-  backupGlobals="false"
-  backupStaticAttributes="false"
-  colors="true"
-  convertErrorsToExceptions="true"
-  convertNoticesToExceptions="true"
-  convertWarningsToExceptions="true"
-  processIsolation="false"
-  stopOnFailure="false"
-  syntaxCheck="false">
+<phpunit colors="true">
   <testsuites>
     <testsuite name="Test-WriteiniFile">
       <directory suffix="Test.php">tests</directory>