Fixed HHVM-175: Only set readConcern when it's 'local' or 'majority'
Fixed HHVM-175: Only set readConcern when it's 'local' or 'majority'

mongodb/mongo-hhvm-driver,jmikola/mongo-hhvm-driver,mongodb-labs/mongo-hhvm-driver,mongodb-labs/mongo-hhvm-driver,jmikola/mongo-hhvm-driver,jmikola/mongo-hhvm-driver-prototype,10gen-labs/mongo-hhvm-driver-prototype,mongodb/mongo-hhvm-driver,mongodb-labs/mongo-hhvm-driver-prototype,jmikola/mongo-hhvm-driver,jmikola/mongo-hhvm-driver,jmikola/mongo-hhvm-driver-prototype,mongodb/mongo-hhvm-driver,mongodb/mongo-hhvm-driver,mongodb-labs/mongo-hhvm-driver,jmikola/mongo-hhvm-driver-prototype,mongodb-labs/mongo-hhvm-driver-prototype,10gen-labs/mongo-hhvm-driver-prototype,mongodb-labs/mongo-hhvm-driver-prototype,mongodb-labs/mongo-hhvm-driver,mongodb-labs/mongo-hhvm-driver-prototype,10gen-labs/mongo-hhvm-driver-prototype,jmikola/mongo-hhvm-driver-prototype,10gen-labs/mongo-hhvm-driver-prototype