Handle missing ~/.gnupg dir

After a fresh `brew install keybase` on OS X 10.11.1, I got the following error:

```
$ keybase login
warn: No config file found; tried '/Users/jgmize/.config/keybase/config.json'
Your keybase username or email: ********************************************
Your keybase login passphrase: ***********************************************
- run scrypt [==================================] 100%
info: Made directory '/Users/jgmize/.cache/keybase'
info: Updated file: /Users/jgmize/.cache/keybase/session.json
info: Made directory '/Users/jgmize/.config/keybase'
info: Updated file: /Users/jgmize/.config/keybase/config.json
error: `gpg` exited with code 2
warn: gpg: keyblock resource `/Users/jgmize/.gnupg/secring.gpg': file open error
warn: gpg: keyblock resource `/Users/jgmize/.gnupg/pubring.gpg': file open error
warn: gpg: fatal: /Users/jgmize/.gnupg: directory does not exist!
warn: secmem usage: 0/0 bytes in 0/0 blocks of pool 0/32768
```

Running `mkdir ~/.gnupg` resolves the issue, but ideally `keybase login` would handle this case automatically.

Start with the `keybase login` flow and its `gpg` invocation; reproduce the failure on a fresh setup where `~/.gnupg` is absent. Done means login handles the missing directory without requiring the user to run `mkdir ~/.gnupg` manually.

Written by the indexing model from the issue text.