Update version of Chosen to v0.12.0

This pull request includes only the changes from running `rake update-chosen`. I noticed the `rake` tasks still reference 0.10.0. What steps are there to make sure the rake tasks are up to date?

````
rake build                         # Build chosen-rails-0.10.0.gem into the pkg directory.
rake install                       # Build and install chosen-rails-0.10.0.gem into system gems.
rake release                       # Create tag v0.10.0 and build and push chosen-rails-0.10.0.gem to Rubygems
rake update-chosen[remote,branch]  # Update with Harvest's Chosen Library
````

[chosen release page](https://github.com/harvesthq/chosen/releases/)