Cannot find module 'sodium' from 'keybase-nacl/lib'

I'm trying to use kbpgp as a module in our js application. Therefor I installed kbpgp via:

```
npm install kbpgp
```

To build the application I use the browserify cli

```
browserify index.js
```

index.js looks like

```
var kbpgp = require('kbpgp')
```

After waiting a few seconds browserify exits with the following failure:

```
Error: Cannot find module 'sodium' from '/home/USER1/Documents/test/node_modules/keybase-nacl/lib'
at /home/USER1/.npm-packages/lib/node_modules/browserify/node_modules/resolve/lib/async.js:46:17
at process (/home/USER1/.npm-packages/lib/node_modules/browserify/node_modules/resolve/lib/async.js:173:43)
at ondir (/home/USER1/.npm-packages/lib/node_modules/browserify/node_modules/resolve/lib/async.js:188:17)
at load (/home/USER1/.npm-packages/lib/node_modules/browserify/node_modules/resolve/lib/async.js:69:43)
at onex (/home/USER1/.npm-packages/lib/node_modules/browserify/node_modules/resolve/lib/async.js:92:31)
at /home/USER1/.npm-packages/lib/node_modules/browserify/node_modules/resolve/lib/async.js:22:47
at FSReqWrap.oncomplete (fs.js:117:15)

➜ test node -v
v6.2.1
➜ test npm -v
3.9.3
```

Start with index.js and reproduce the failure using browserify index.js with the reported Node and npm versions. Inspect how kbpgp and keybase-nacl resolve the sodium dependency; done means the same entry point bundles without the missing-module error.

Written by the indexing model from the issue text.