Fix mkdirP ternary to work with no outputDir

As the comment says, we don't want to call mkdirP(null); the logic here was reversed. The old code worked before because mkdirP(manifestDir) was executed before anything was written, but this code is what was originally intended.
Merge pull request #56 from spicyj/mkdirp

Fix mkdirP ternary to work with no outputDir.