`.time` is deprecated since https://github.com/esphome/esphome/pull/531; this updates the reference in the documentation so that warnings like the following can be avoided:
```
src/main.cpp: In lambda function:
src/main.cpp:462:20: warning: 'esphome::time::ESPTime::<anonymous union>::time' is deprecated (declared at src/esphome/components/time/real_time_clock.h:35): .time is deprecated, use .timestamp instead [-Wdeprecated-declarations]
return (time.time);
```
* Update time.rst to add GPS as source
Just saw that GPS was missing in the time documentation as a time source (the GPS component documentation redirect to the time page for configuration details)
* travis
