Drop PHP 5.5 support

According to our policy to support PHP version 6 months after their
end of life, it is time to drop PHP 5.5.

.travis.yml

@@ -1,7 +1,6 @@
 language: php
 
 php:
-  - 5.5
   - 5.6
   - 7.0
   - 7.1

README.md

@@ -33,7 +33,7 @@ PhpSpreadsheet is a library written in pure PHP and providing a set of classes t
 
 ## Requirements
 
- * PHP version 5.5 or higher
+ * PHP version 5.6 or higher
  * PHP extension php_zip enabled (required if you need PhpSpreadsheet to handle .xlsx .ods or .gnumeric files)
  * PHP extension php_xml enabled
  * PHP extension php_gd2 enabled (optional, but required for exact column width autocalculation)

composer.json

@@ -23,7 +23,7 @@
         }
     ],
     "require": {
-        "php": "^5.5|^7.0",
+        "php": "^5.6|^7.0",
         "ext-mbstring": "*",
         "ext-iconv": "*",
         "ext-xml": "*",

docs/index.md

@@ -12,7 +12,7 @@ spreadsheet file formats, like Excel and LibreOffice Calc.
 
 The following software is required to develop using PhpSpreadsheet:
 
--   PHP version 5.5 or newer
+-   PHP version 5.6 or newer
 -   PHP extension php\_zip enabled (see
     [FAQ](./faq.md#php-complains-about-ziparchive-not-being-found))
 -   PHP extension php\_xml enabled

samples/index.php

@@ -3,7 +3,7 @@
 require_once 'Header.php';
 
 $requirements = [
-    'PHP 5.5.0' => version_compare(phpversion(), '5.5.0', '>='),
+    'PHP 5.6.0' => version_compare(phpversion(), '5.6.0', '>='),
     'PHP extension XML' => extension_loaded('xml'),
     'PHP extension xmlwriter' => extension_loaded('xmlwriter'),
     'PHP extension mbstring' => extension_loaded('mbstring'),