Prep for 3.0.0.rc

I know we haven't got 2.3.0 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
- [x] ship 2.3.0.rc2
- [x] ship 2.3.0.rc3
- [x] create 2.3-stable branch
- [x] make a note in master README
- [x] merge in 1.9.3 requirement (#949)
- [x] merge in Rails 3.2.13 requirement (#950)
- [x] remove deprecated `TimeInput`,  `DateInput`, `DateTimeInput` (#1024)
- [x] remove deprecated option `:input_html => { :value => '...'}` (#1025)
- [x] remove deprecated option `:hint_class` (#1025)
- [x] remove deprecated option `:error_class` (#1025)
- [x] remove deprecated option `:group_by` (#1025)
- [x] remove deprecated option `:group_label` (#1025)
- [x] remove deprecated option `:find_options` (#1025)
- [x] simplify travis matrix and include Ruby 2.1 (#1026)

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.