mkdirp should be a prodcution dependency

[arieljannai]

caused the applications.json not to be created and the file-finder not to work (tinytacoteam/zazu#194)

[twang2218]

LGTM.

I can repeat the issue https://github.com/tinytacoteam/zazu/issues/194 by:

• npm i --production on master branch;
• node appCache.js '{}', then got the exception:

module.js:472
    throw err;
    ^

Error: Cannot find module 'mkdirp'
    at Function.Module._resolveFilename (module.js:470:15)
    at Function.Module._load (module.js:418:25)
    at Module.require (module.js:498:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Users/taowang/.zazu/plugins/tinytacoteam/zazu-file-finder/lib/appInfo.js:6:16)
    at Module._compile (module.js:571:32)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)

And tested this PR, everything works now.

[bayleedev]

Wow, thanks!