FIX: Let `Octopus::RelationProxy#select` takes a block so it can be used just like `Array#select`.

**Problem:** You can't use the block form of `Octopus::RelationProxy#select` to make it behave like `Array#select`. A completely valid query like this fails:

```ruby
User.using(:brazil).where(first_name: "Evan").select{|u| u.last_name ~= /mor/}
```

It raises: `ArgumentError: Call this with at least one field`

**Cause:** I forgot to pass through `&block` on the delegated `select` method on `Octopus::RelationProxy`.

**Pull Request:** If only [e95304d](https://github.com/emorikawa/octopus/commit/e95304d6087bad11a74d44843f99d4c0819f15de) is applied, then the one test will fail. [30ce449](https://github.com/emorikawa/octopus/commit/30ce449c6bdc45570b166c3dea03ae39d295cbed) should bring everything back to green.