Peg rails dependency to 4.0.0

### WHAT

* Set rails in gem to at least 4.0.0
* Also fix some deprecation warnings in the tests

### WHY

`bundle` didn't work for me when I initially checked out the gem:

```
Bundler could not find compatible versions for gem "activerecord":
  In Gemfile:
    rails (>= 0) ruby depends on
      activerecord (= 4.1.0.beta) ruby

    activerecord (4.0.0.rc1)

Could not find gem 'activerecord-deprecated_finders (~> 0.0.3) ruby', which is required by gem 'activerecord (~> 4.0.0.beta) ruby', in any of the sources.
```

Now that `4.0.0` we might as well point to that. After I did this, I get one failing test in `SaveFromAfterCommitBlockTest#test_after_commit_in_save`, but I'm not sure if it existed before these commits? (can't find travis build)