Install failure on Mac OS X

On OS X 10.10.5, the homebrew beta install works fine, but the npm install fails (and I'd rather use real keybase than kbstage forever).

`npm-install -g keybase-installer` completes successfully, but `keybase-installer` fails with:

```
child_process.js:1162
throw errnoException(err, 'spawn');
^
Error: spawn ENOTDIR
at exports._errnoException (util.js:746:11)
at ChildProcess.spawn (child_process.js:1162:11)
at exports.spawn (child_process.js:995:9)
at SpawnEngine.exports.SpawnEngine.SpawnEngine._spawn (/usr/local/lib/node_modules/keybase-installer/node_modules/iced-spawn/lib/cmd.js:162:26)
at /usr/local/lib/node_modules/keybase-installer/node_modules/iced-spawn/lib/cmd.js:282:17
at Deferrals.exports.Deferrals.Deferrals._call (/usr/local/lib/node_modules/keybase-installer/node_modules/iced-runtime/lib/runtime.js:86:16)
at /usr/local/lib/node_modules/keybase-installer/node_modules/iced-runtime/lib/runtime.js:98:26
at exports.trampoline.trampoline (/usr/local/lib/node_modules/keybase-installer/node_modules/iced-runtime/lib/runtime.js:64:14)
at Deferrals.exports.Deferrals.Deferrals._fulfill (/usr/local/lib/node_modules/keybase-installer/node_modules/iced-runtime/lib/runtime.js:96:16)
at /usr/local/lib/node_modules/keybase-installer/node_modules/iced-spawn/lib/cmd.js:277:28
```

Start by reproducing the npm installation on OS X 10.10.5 with `npm install -g keybase-installer` and then `keybase-installer`. Trace the failure from the `keybase-installer` entry point through `node_modules/iced-spawn/lib/cmd.js`; done means the installer runs successfully instead of raising `spawn ENOTDIR`.

Written by the indexing model from the issue text.