Update libmemcached.zep

Zephir\CompilerException: Variable 'servers' cannot be read because it's not initialized in /home/ikhrustalev/Desktop/work/cphalcon/phalcon/session/adapter/libmemcached.zep on line 87

	   ["servers": servers, "client": client, "prefix": prefix]
	----------------------^
Merge pull request #2525 from rznzippy/patch-2

Update libmemcached.zep