[related #8] manually expire cache

downcache just seems to keep things forever :(
[fixes #6] `m available`

for listing available mongo versions

```
Usage:
  m available [--stable --unstable --all --pokemon]

Options:
  --stable      Only list available stable versions.
  --unstable    Only list available unstable versions.
  --rc          Include release candidates in available versions.
  --pokemon     Show all available versions you do not have installed
so you can collect all the MongoDB's.
```