Force pull secret key doesn't overwrite in gpg keyring

`-f, --force force repull from server even if it's already stored locally`

Key downloads, but import fails with gpg complaining that it already has the secret key and so it's leaving it unchanged.

```
debug: Check key secret: {"remote":true,"local":true}
debug: Check key public: {"remote":true,"local":true}
debug: -- KeyPull::load_user -> null 1
debug: ++ KeyPull::secret_pull
Your keybase login passphrase (to pull your private key from the server): *
- run scrypt [==================================] 100%
debug: || Save key /################ to master keyring
debug: || Call to gpg: { args: [ '--import', '--import-options', 'import-local-sigs' ],
debug: stdin: '',
debug: quiet: true,
debug: secret: true }
warn: gpg: key ########: already in secret keyring
warn: gpg: Total number processed: 1
warn: gpg: secret keys read: 1
warn: gpg: secret keys unchanged: 1
error: `gpg` exited with code 2
```

butlertron-osx:src litui$ gpg --version
gpg (GnuPG/MacGPG2) 2.0.22
libgcrypt 1.5.3

butlertron-osx:src litui$ keybase --version
0.0.34

Start with the `-f, --force` key-pull path and the `gpg --import --import-options import-local-sigs` invocation shown in the report; reproduce it with GnuPG 2.0.22 and keybase 0.0.34. Done means a forced repull can replace the existing secret key in the local keyring without the import failing or leaving it unchanged.

Written by the indexing model from the issue text.