fix loading of configuration file

my directory structure is the following

bin/phpdoc
build/phpdoc_api.xml

before the fix this two variants works:
bin$ ./phpdoc -c ./../build/phpdoc_api.xml --force
build$ ./../bin/phpdoc -c phpdoc_api.xml --force 

after the fix also this works:
$ ./bin/phpdoc -c ./build/phpdoc_api.xml --force
Merge pull request #1375 from gimler/patch-3

fix loading of configuration file