Change .ssh directory permissions to 700

The ssh(1) manpage recommends to set permissions for the $HOME/.ssh/ directory to 700:

```
     ~/.ssh/
             This directory is the default location for all user-specific configuration and authentication informa‐
             tion.  There is no general requirement to keep the entire contents of this directory secret, but the
             recommended permissions are read/write/execute for the user, and not accessible by others.
```

I consider it a good idea to follow this recommendation as it makes it just a little bit more secure.