m <version> doesn't work as expected

According to `m --help`

```
  m <version>                    # Install or activate MongoDB <version>
```

But it only prints out the version, it doesn't actually switch or install it. Example: 

``` sh
tr@enter:~$ m
  ο 2.6.6
  2.8.0-rc4

tr@enter:~$ m 2.6.5
2.6.5

tr@enter:~$ m
  ο 2.6.6
  2.8.0-rc4

tr@enter:~$ m 2.8.0-rc4
2.8.0-rc4

tr@enter:~$ m
  ο 2.6.6
  2.8.0-rc4

```
