Update MongoSession.php

"safe" is now deprecated....

http://php.net/manual/en/mongocollection.remove.php

i think you can use $options without error but send "true" is a fatal error now...
Merge pull request #8 from UnSleep/patch-2

Update MongoSession.php to swap out safe parameter for new WriteConcerns option as of PECL mongo >=0.9.0.