Allow aeson >= 0.7

Related to #3, again.
Thanks for detailed comment!
I agree with the reason that converting everything which fits to `Int64` to `Int64` is better.

As for large numbers, I agree with throwing totality away is a big minus. But, as I can consider, if `numberConversion` has type signature `Scientific -> BSON.Value`, there's few choices left for the user, just throw exception or coerce to the limit. 

So what if we have safe one which has `AESON.Value -> Maybe BSON.Value` and not total one which has `AESON.Value -> BSON.Value`? By this, user can choose what to do with big number.