FIX: Selecting specific fields was broken for all `RelationProxy`s

**Problem:** `User.where(name: "foo").select(:id)` and all [similar queries](http://guides.rubyonrails.org/active_record_querying.html#selecting-specific-fields) that used an `ActiveRecord::Relation` + [`#select`](http://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-select) were broken.

**Cause:** This is because when `#select` was called on `Octopus::RelationProxy`, the [`Kernel#select`](http://www.ruby-doc.org/core-2.1.2/Kernel.html#method-i-select) method was erroneously called instead of `method_missing`

**Solution:** A `Octopus::RelationProxy#select` method was added that delegates to `method_missing`. This is similar to `#inspect`, `#==`, and `#as_json`.

**Pull Request:** I broke the tests & the fix into two different commits. If you run the tests on [667b913d](https://github.com/emorikawa/octopus/commit/667b913d1fab256e0cb259c6306ac96755f6f041) without the fix you will see the described errors. The next commit at [2474fe20](https://github.com/emorikawa/octopus/commit/2474fe208f9968b2f6c468daa9ee0bd619eb9100) will bring everything back to green.