Added explicit UTF8 string encoding to Rakefile.

Was getting the following error:

```
 rake bootstrap --trace
rake aborted!
rake aborted!
ArgumentError: invalid byte sequence in US-ASCII
```

Found the answer [here](http://stackoverflow.com/questions/17031651/invalid-byte-sequence-in-us-ascii-argument-error-when-i-run-rake-dbseed-in-ra). Seems to work.