Fix grunt requirejs builds

Summary:
RequireJS build now works, but only builds the initial includes, not files that are loaded dynamically later.

- Builds Init.min.js
- Build require.min.js as part of the task
- Add .gitignore for built js files

Todo
- Minify other requires that are still being loaded

Test Plan:
- Run `grunt requirejs` and see that it build successfully
- Switch to production env, include Init.min.js and see that it works

Reviewers: shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T213

Differential Revision: https://phabricator.ushahidi.com/D76