Updates to BulkWriteResult modified count is now Try[Int]

Depending on topology with pre 2.6 servers modifiedCount could
return null from the server.  The Java driver adds a check so accessing
modifiedCount where the result was null would throw an UnsupportedOperationException
exception.  So access in Casbah wraps the modifiedCount in a Try.

SCALA-146 SCALA-147 SCALA-148
Added BulkWriteException wrapper

Wrapped the BulkWriteException because it can contain a BulkWriteResult SCALA-116
Bump to Casbah-2.7.0-RC2