allow boxen runs to continue without saving config

From what I can tell, Boxen will always fail when it can't access the OS X keychain, even when its possible to continue running. I hit this issue when trying to run `boxen --token abc12345...` after ssh-ing in. When that happens, I see the following:

```
Boxen Keychain Helper: Encountered error code: -25308
Error: User interaction is not allowed.
Boxen Keychain Helper: Encountered error code: -25308
Error: User interaction is not allowed.
Boxen Keychain Helper: Encountered error code: -25308
Error: User interaction is not allowed.
/opt/boxen/repo/.bundle/ruby/2.0.0/gems/boxen-2.6.0/lib/boxen/keychain.rb:48:in `set': Can't save GitHub API Token in the keychain. (Boxen::Error)
        from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/boxen-2.6.0/lib/boxen/keychain.rb:30:in `token='
        from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/boxen-2.6.0/lib/boxen/config.rb:73:in `save'
        from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/boxen-2.6.0/lib/boxen/cli.rb:48:in `run'
        from /opt/boxen/bin/boxen:80:in `<main>'
```

Even though its possible for Boxen to run successfully without saving.

My solution is to change a failed save to a warning and move forward. Perhaps its possible to do this some other way; if so, please let me know. Another approach might be a flag that allows Boxen to ignore this when set, but continuing on if possible seems like a better default to me, which is why I implemented it this way.