[Config] Don't check PATH_CACHE for memcached

[jakubvalenta]

Motivation

Be able to run RSS-Bridge from a read-only directory with memcached cache.

Changes

Move the check for PATH_CACHE from Configuration to the respective cache implementations.

Also gitignore .phpunit.result.cache which is created when running phpunit as described in the Coding style policy.

Testing

1. Check that file cache fails when PATH_CACHE is not writable:

   $ chmod -w cache
   $ echo "[cache]
   type=\"file\"" > config.ini.php
   $ php index.php action=display bridge=Twitter context=By+username u=jack norep=on format=Atom
   Exception: RSS-Bridge does not have write permissions for /home/jakub/devel/rss-bridge/lib/../cache/! in /home/jakub/devel/rss-bridge/lib/error.php:24
   #0 /home/jakub/devel/rss-bridge/lib/error.php(42): returnError()
   #1 /home/jakub/devel/rss-bridge/caches/FileCache.php(13): returnServerError()

2. Check that sqlite cache fails when PATH_CACHE is not writable:

   $ chmod -w cache
   $ echo "[cache]
   type=\"sqlite\"" > config.ini.php
   $ php index.php action=display bridge=Twitter context=By+username u=jack norep=on format=Atom
   Exception: RSS-Bridge does not have write permissions for /home/jakub/devel/rss-bridge/lib/../cache/! in /home/jakub/devel/rss-bridge/lib/error.php:24
   Stack trace:
   #0 /home/jakub/devel/rss-bridge/lib/error.php(42): returnError()
   #1 /home/jakub/devel/rss-bridge/caches/SQLiteCache.php(19): returnServerError()
   ...

3. Check that memcached doesn't fail when PATH_CACHE is not writable:

   $ chmod -w cache
   $ echo "[cache]
   type=\"memcached\"" > config.ini.php
   $ php index.php action=display bridge=Twitter context=By+username u=jack norep=on format=Atom
   <?xml version="1.0" encoding="UTF-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom">
   ...

[somini]

LGTM, thanks for the very detailed merge request (with examples!).

[dvikan]

Looks good.