Typo: Check for puppet-lint, not puppet

This was just a typo and prevents puppet syntax checking from working if
`puppet-lint` is installed but `puppet` isn't.

`puppet` isn't needed for `puppet-lint` to work, assuming we're talking
about [rodjek's `puppet-lint`](https://github.com/rodjek/puppet-lint/)

And, no, I don't particularly like puppet; but syntastic makes working
with puppet much much easier.