Without `User` in the global namespace, the tests fail.

```diff
-class User < ActiveRecord::Base
-  self.table_name = "users"
-end
-
 class Barn::Builders::ActiveRecordTest < MiniTest::Test
+  class User < ActiveRecord::Base
+    self.table_name = "users"
+  end
+
```

```
  1) Error:
Barn::Builders::ActiveRecordTest#test_activerecord:
NoMethodError: undefined method `ancestors' for nil:NilClass
    /Users/simeon/Projects/barn/lib/barn/builders/activerecord.rb:40:in `call'
    /Users/simeon/Projects/barn/lib/barn/namespace.rb:17:in `build'
    test/barn/builders/activerecord_test.rb:26:in `test_activerecord'
```

[`#constantize`](https://github.com/jch/barn/blob/3cd593e1d9062dff20424f169ca9d1c02a19c60e/lib/barn/builders/activerecord.rb#L34) can't find the namespaced `User`.  

`ActiveSupport::Inflector#constantize` [looks for the constant on `Object`](https://github.com/rails/rails/blob/9e0b3fc7cfba43af55377488f991348e2de24515/activesupport/lib/active_support/inflector/methods.rb#L228-L231).

Borrowing @achiu's [idea](https://github.com/jch/barn/commit/aad1e68bdcf962fb2f06c0ce9de934d12685d19b), we can pass context into builders, and look for constants in that context.  I'll push my thoughts.