Removed default event value from being set to '1'

This behavior is different between the back-end tracking integration and the front-end: https://github.com/segmentio/analytics.js/blob/master/lib/integrations/google-analytics.js#L117

I personally prefer it not to default to anything, because it screws up our value tracking on events that don't have a dollar value, but either way the two libraries should remain consistent.