Update mongodb driver version

[BobHarper1]

I found that in attempting to import a MongoDB collection the following error in the CLI where I had run dpd

========================================================================================
=  Please ensure that you set the default safe variable to one of the                  =
=   allowed values of [true | false | {j:true} | {w:n, wtimeout:n} | {fsync:true}]     =
=   the default value is false which means the driver receives does not                =
=   return the information of the success/error of the insert/update/remove            =
=                                                                                      =
=   ex: new Db(new Server('localhost', 27017), {safe:false})                           =
=                                                                                      =
=   http://www.mongodb.org/display/DOCS/getLastError+Command                           =
=                                                                                      =
=  The default of false will change to true in the near future                         =
=                                                                                      =
=  This message will disappear when the default safe is set on the driver Db           =
========================================================================================

I think this is caused by a mismatch between version 2.x.x of the mongodb node package (i.e. the driver) used by deployed and the dependency version of the driver in dpd-importer. Updating the dependency in dpd-importer solved this for me.

I also realise that in order to import a collection from a MongoDB server the deployd instance needs to have the same collection and properties present in the resources, or the import fails. So added this information to readme.