Add basic integration testing with ActiveRecord

This can certainly be expanded upon but this is the leanest method I have found to test with real ActiveRecord while still isolating the unit tests.

Expected to fail on Rails 3.0 due to ludicast/yaml_db#37 with the error
```
ActiveRecord::StatementInvalid:
  TypeError: can't convert Arel::SelectManager into String
```
or
```
ActiveRecord::StatementInvalid:
  TypeError: no implicit conversion of Arel::SelectManager into String
```
and on Rails >=4.0 due to #3/#6 with the error
```
NoMethodError:
  undefined method `[]=' for #<ActiveRecord::Result:0x00000003390528>
```