Change PlaceInstruction.selection_id type to int
Change PlaceInstruction.selection_id type to int

Previously PlaceInstruction.selection_id was declared to be a float, however the
API documentation says that it is a long:

https://api.developer.betfair.com/services/webapps/docs/x/SIA6#BettingTypeDefinitions-PlaceInstruction

This changes the declared data type in the model to int. int is used
over long as this maintains Python 3 compatability (in Python 2
int('<some number>') will return a long if the number is big enough
to require it).

skozilla/betfair.py,jmcarp/betfair.py,Taketrung/betfair.py