Update bower.json

Added "main" for support with rails-assets as per:

> Post-install message from rails-assets-select2-bootstrap:
> This component doesn't define main assets in bower.json.
> Please open new pull request in component's repository:
> https://github.com/MattKetmo/select2-bootstrap-css
Merge pull request #1 from jufemaiz/master

Update bower.json