Lock ruby version to 1.9.3.

Because of https://github.com/kandanapp/kandan-count/blob/master/kandan-count.gemspec#L16

Also, When I push to Heroku:

```bash
       Gem::InstallError: kandan-count requires Ruby version >= 1.9.3.
       An error occurred while installing kandan-count (1.1.0), and Bundler cannot continue.
       Make sure that `gem install kandan-count -v '1.1.0'` succeeds before bundling.
 !
 !     Failed to install gems via Bundler.
 !
 !     Heroku push rejected, failed to compile Ruby/rails app
```