Error and stack trace on `keybase prove twitter`

On OSX 10.10, with (as the stacktrace shows) node installed via nvm, if that makes a difference.

```
09:13 $ keybase prove twitter
Your username on Twitter: jzetlen
/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/lib/command/prove.js:578
_this.gen.run(esc(__iced_deferrals.defer({
^
TypeError: Cannot read property 'run' of null
at iced.Deferrals.parent (/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/lib/command/prove.js:578:40)
at /Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/lib/command/prove.js:582:31
at Deferrals.exports.Deferrals.Deferrals._call (/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:86:16)
at /Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:98:26
at exports.trampoline.trampoline (/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:64:14)
at Deferrals.exports.Deferrals.Deferrals._fulfill (/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:96:16)
at ret (/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:29:18)
at /Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-error/index.js:92:22
at /Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/lib/command/proof_base.js:178:18
at Deferrals.exports.Deferrals.Deferrals._call (/Users/james_zetlen/.nvm/versions/node/v0.12.5/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:86:16)
```

Reproduce `keybase prove twitter` on OSX 10.10 with the reported nvm-installed Node version, then inspect `lib/command/prove.js` around line 578 and the related flow in `lib/command/proof_base.js`. Done means the Twitter proof command no longer raises `Cannot read property 'run' of null` and reaches its normal completion path.

Written by the indexing model from the issue text.