Bump version

I know we haven't got 3.2.0 out yet, but I'm losing track of a few things in my head so here's a branch to start bring it together.

- [x] bump version
- [ ] ship 2.3.0.rc2
- [ ] create 2.3-stable branch
- [ ] make a note in master README
- [ ] ship 2.3.0 final
- [ ] merge in 1.9.3 requirement
- [ ] merge in Rails 3.2.13 requirement
- [ ] remove deprecated option `:input_html => { :value => '...'}`
- [ ] remove deprecated option `:hint_class`
- [ ] remove deprecated option `:error_class`
- [ ] remove deprecated option `:group_by`
- [ ] remove deprecated option `:group_label`
- [ ] remove deprecated option `:find_options`
- [ ] remove deprecated TimeInput
- [ ] remove deprecated DateInput
- [ ] remove deprecated DateTimeInput

It'd be interesting to explore 3.0 as a "Clean shit up only" release, but I'm reviewing open PRs for features too, and ideally bug fixes for open issues would be applied to 2.3-stable as well.