Added gitkeep file to keep the build folder

[teameh]

README.md does not mention that users have to create the /build folder before the first npm run make run. I got this when trying to build:

$ npm run make-dev

> react-php-v8js@1.0.0 make-dev /Users/tieme/react-php-v8js/example
> browserify -t [ envify --NODE_ENV development ] src/react-bundle.js > build/react-bundle.js

sh: build/react-bundle.js: No such file or directory

npm ERR! Darwin 15.3.0
npm ERR! argv "/usr/local/Cellar/node/0.12.5/bin/node" "/usr/local/bin/npm" "run" "make-dev"
npm ERR! node v5.6.0
npm ERR! npm  v3.7.5
npm ERR! code ELIFECYCLE
npm ERR! react-php-v8js@1.0.0 make-dev: `browserify -t [ envify --NODE_ENV development ] src/react-bundle.js > build/react-bundle.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the react-php-v8js@1.0.0 make-dev script 'browserify -t [ envify --NODE_ENV development ] src/react-bundle.js > build/react-bundle.js'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the react-php-v8js package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     browserify -t [ envify --NODE_ENV development ] src/react-bundle.js > build/react-bundle.js
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs react-php-v8js
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls react-php-v8js
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR!     /Users/tieme/react-php-v8js/example/npm-debug.log

A .gitkeep file will add the folder to the repo, fixing this issue for users who quickly want to try out the project :)

[MickeyKay]

Thank you! Just saved my butt. Would love to see this merged.

[zpao]

Thanks!