Fix problem creating nested directories with no parent

[maxcnunes]

I was getting this error before my fixing:

- Running electron-builder 2.2.0
- Ouput directory ´/Users/maxcnunes/Development/sqlectron/sqlectron-gui/dist/installers/osx/´ does not exist
fs.js:747
  return binding.mkdir(pathModule._makeLong(path),
                 ^
Error: ENOENT, no such file or directory '/Users/maxcnunes/Development/sqlectron/sqlectron-gui/dist/installers/osx/'
    at Error (native)
    at Object.fs.mkdirSync (fs.js:747:18)
    at Object.Builder.build (/Users/maxcnunes/Development/sqlectron/sqlectron-gui/node_modules/electron-builder/index.js:45:10)
    at Object.<anonymous> (/Users/maxcnunes/Development/sqlectron/sqlectron-gui/node_modules/electron-builder/cli.js:36:9)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:501:10)
    at startup (node.js:129:16)

[mention-bot]

By analyzing the blame information on this pull request, we identified @stefanjudis and @etiktin to be potential reviewers.

[stefanjudis]

Thanks. LGTM. 👍