The `snap interfaces` command prints out a deprecation notice to stderr. This breaks the completion
support for interfaces, connect and disconnect commands like so:
```
$ snap connect <TAB>error: no interfaces found
error: no interfaces found
...
'snap interfaces' is deprecated; use 'snap connections'.
'snap interfaces' is deprecated; use 'snap connections'.
'snap interfaces' is deprecated; use 'snap connections'.
error: no interfaces found
error: no interfaces found
'snap interfaces' is deprecated; use 'snap connections'.
```
Ultimately, the snap command completion should switch to `snap connections`. However, for now try to
workaround the notice by redirecting stderr.
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>