`keybase login`, Use GPG, does not use 'default-key' setting

At the moment I have two signing subkeys - one is about to expire, so I created a new one. I would prefer to use my new subkey to sign installations; but keybase uses the old one.

My gpg.conf includes a line instructing GPG to use my new signing subkey by default:

```
default-key E99F65E6
```

I ran these steps with keybase 1.0.11-20160211170054+0b33bc6:
- keybase login
- (4) Use GPG
- (1) Keybase can use GPG commands to sign the installation.
- Choose a key: [I selected my master key id, 8FB4E35FE86B913B, which was the only option]

At this point I was prompted to enter a passphrase to unlock my old signing subkey, A5CC2BE3.

Everything works - but I don't know what will happen to the the validity of my device in a few days when my old signing subkey expires.

my log id: cec2262fc87d9cb1a1bab71c

Start by reproducing the `keybase login` flow with the reported keybase 1.0.11 version, GPG enabled, and a `gpg.conf` containing `default-key E99F65E6`. Trace how the selected master key leads to signing subkey A5CC2BE3, then verify that the default-key setting is respected and that the installation remains valid after the old subkey expires.

Written by the indexing model from the issue text.