Debian packaging improvements

Some small changes to make the Riemann deb nicer to work with.
It appears to do the right thing, assuming the right thing is "nothing":

```
# dpkg -r riemann
(Reading database ... 310096 files and directories currently installed.)
Removing riemann (0.2.6) ...
# service riemann status; echo $?
0
# service riemann stop; echo $?
0
# service riemann start; echo $?
0
```