Specs added for the case when model has counter_cache field

PS: gem with the version 0.1.1 that's hosted on rubygems.org seems to have old gemspec file with broken rails dependency:

```
➜  xxxxx git:(master) ✗ ruby -v
ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin13.0.1]
➜  xxxxx git:(master) ✗ bundle update
Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    custom_counter_cache (~> 0.1.1) ruby depends on
      rails (~> 3.1) ruby

    rails (4.0.2)
```