Bump minimum required version of activerecord

It seems not working with activerecord 3.0 or 3.1.

```
% bundle show | grep activerecord
  * activerecord (3.1.12)
% bundle exec rake
Run options: --seed 62813

# Running:

EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE

Finished in 0.094504s, 1068.7419 runs/s, 0.0000 assertions/s.

  1) Error:
ZeroBasedTest#test_reordering:
NoMethodError: undefined method `schema_cache' for #<ActiveRecord::ConnectionAdapters::SQLite3Adapter:0x007f9eeee1f070>
    /home/eagletmt/.ghq/github.com/swanandp/acts_as_list/test/test_list.rb:18:in `setup_db'
    /home/eagletmt/.ghq/github.com/swanandp/acts_as_list/test/test_list.rb:109:in `setup'

(snip)
```