Sort dyno list properly with >=10 dynos, fixes #53

Before:

```
$ hk -a mytestapp ls dynos
check.1
check.10
check.11
check.12
check.13
check.14
check.15
check.16
check.17
check.18
check.19
check.2
check.20
check.3
check.4
check.5
check.6
check.7
check.8
check.9
```

After:

```
$ hk -a mytestapp ls dynos
check.1
check.2
check.3
check.4
check.5
check.6
check.7
check.8
check.9
check.10
check.11
check.12
check.13
check.14
check.15
check.16
check.17
check.18
check.19
check.20
```