correct usage of batch update in unit test

because of the isolation level (which I should investigate more in the near future), it's "bad practice" to fire update query during a "find".
ideally the "awaitable" in c# or facebook-hack should be introduced to python so we can do
```
update_gens = []
for instance in model.find():
  update_gens.push(instance.save())
await genv(update_gens)
```
where `genv` is a batch call to a list of yields, and @save() should return a yield instead of direct function call.
remove python 2.6 from travis setup