run grunt-docker with grunt 0.4.x

- Fixed the task-code to run with **grunt 0.4.x**
- renamed `grunt.js`to `Gruntfile.js`
- updated `package.json` to match **grunt 0.4.x**
- increased version
- updated readme
- updated travis.yml

I also checked if the tasks/tests in `grunt.js` work well.