Stop reporting appveyor on_success, because it's noisy
Appveyor currently reports:
```
on_success:
- cmd: ... success
on_failure:
- cmd: ... failure
```
which is really noisy.
Travis currently reports:
```
irc:
...
on_success: change
on_failure: change
```
which seems ok.
We should make Appveyor notifications more like Travis.
issue