Cannot spell check cotnent, mdspell is not installed

I just cloned. the repo and installed the Node dependencies as instructed in the README.

When I run `npm run test` I get an error about mdspell not being found:

```
> revealjs.com@1.0.0 test
> mdspell -r -n -a --en-us 'src/*.md' 'src/**/*.md'

sh: mdspell: command not found
```

It appears that the `markdown-spellcheck` package wasn't included in package.json