use Context stage to identify stage, not function name

According to http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html

There is actually a 'stage' attribute in the context of each lambda call. Instead of ripping apart the function name, would be much cleaner to use this.

How it's currently done here

        # Get the version of the api
        try:
            self.version = context.function_name.split('-')[1] #grab the stage version name from the function name
        except AttributeError, e:
            raise AWSError({'error' : 'Fatal Error'})

how it should be done

        # Get the version of the api
        try:
            self.version = context.stage #grab the stage version name from the function name
        except AttributeError, e:
            raise AWSError({'error' : 'Fatal Error'})

Saasli / saasli-backend

use Context stage to identify stage, not function name #38