prevent buffer-browserify from getting included to reduce bundle size

this uses the `Base64` module which is a smaller implementation than the previously used buffer-browserify one

it also upgrades `concat-stream` which now uses `bops.is` to check for bufferness instead of the previous `Buffer.isBuffer` which caused buffer-browserify to get included

before:

```sh
browserify index.js | wc -c
   86428
```

after:
```sh
browserify index.js | wc -c
   45525
```