Fixed a small typo in the devDepdency declaration for gulp

[BrandTurner]

Hey guys,

Thanks for supporting this fantastic application!

After cloning, I tried running npm install and ran into this error:

➜ npm install
npm ERR! code 1
npm ERR! Command failed: /usr/local/bin/git checkout 4.0
npm ERR! error: pathspec '4.0' did not match any file(s) known to git.
npm ERR!

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/USER/.npm/_logs/20*******debug.log