require-dev addition of doctrine/cache for testing

People who use the Doctrine Adaptor should have Doctrine in their own
composer file, and we shouldn’t make doctrine a dependency for projects
that don’t need it. This is a step in that direction that also allows
us to still test the Doctrine Adaptor.

composer.json

@@ -24,7 +24,8 @@
     "require-dev": {
         "phpunit/phpunit": "4.1.*",
         "fabpot/php-cs-fixer": "0.4.0",
-        "satooshi/php-coveralls": "dev-master"
+        "satooshi/php-coveralls": "dev-master",
+        "doctrine/cache": "~1.0"
     },
   "autoload": {
     "psr-0": {"Tedivm\\StashBundle": ""}