Avoid potentially unsupported PSR-16 cache keys

Ensure compatibility with all PSR-16 cache implementation by using
a stricter character set for cache key that match the spec.

Fixes #354

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [Unreleased]
+
+### Fixed
+
+- Avoid potentially unsupported PSR-16 cache keys - [#354](https://github.com/PHPOffice/PhpSpreadsheet/issues/354) 
+
 ## [1.1.0] - 2018-01-28
 
 ### Added

src/PhpSpreadsheet/Collection/Cells.php

@@ -285,7 +285,7 @@ class Cells
      */
     private function getUniqueID()
     {
-        return uniqid('phpspreadsheet-', true) . '-';
+        return uniqid('phpspreadsheet.', true) . '.';
     }
 
     /**