Display trips departing in 1 and -1 minutes

This PR updates the departure time logic to only show trips departing at `0` as `NOW`. This change is being made to improve the departure accuracy as well as match how the Android app displays departure times. Currently trips departing in `1` or `-1` minutes are shown as `NOW` which can result in a trip saying it has both departed and is departing now as can be seen here:

![image](https://f.cloud.github.com/assets/1192780/1168232/fd7f7f2a-209c-11e3-8545-35c81d2ec97c.jpg)