Fastboot: md5 is not defined

Hello,

It looks like there's an error when running in Fastboot. The [md5 shim](https://github.com/johnotander/ember-cli-gravatar/blob/master/vendor/ember-cli-gravatar/md5-shim.js#L7) throws an error that md5 is not defined on line 7. This creates an infinite loop and crashes the fastboot server.

```
There was an error running your app in fastboot. More info about the error:
ReferenceError: md5 is not defined
at Module.callback (/Users/ryan/projects/ember-map-frontend/tmp/broccoli_persistent_filterautoprefixer_filter-output_path-xC9ntuEm.tmp/assets/vendor/ember-cli-gravatar/md5-shim.js:7:1)
```

```
/Users/ryan/projects/ember-map-frontend/tmp/broccoli_persistent_filterautoprefixer_filter-output_path-xC9ntuEm.tmp/assets/vendor.js:27980
throw new Error('infinite rendering invalidation detected');
^
Error: infinite rendering invalidation detected
```