Fix issue with "rake docs"

When I run "rake docs" I get this error:

```
rm -rf .yardoc
rm -rf api-docs
SOURCE=1 bundle exec yard
rake aborted!
Command failed with status (127): [SOURCE=1 bundle exec yard...]
tasks/docs.rake:33:in `block in <top (required)>'
Tasks: TOP => docs
(See full trace by running task with --trace)
```

With this fix I'm now able to run "rake docs" and generate documentation
locally.
Merge pull request #109 from jamesls/rake-docs-fix

Fix issue with "rake docs"