Test against Rails 4.1

`BUNDLE_GEMFILE=Gemfile.rails4.1 bundle exec rspec spec`
Rails 4.1: remove unnecessary scoped in test
Rails 4.1: deprecated_finders for apply_finder_options
Test with Rails 4.0.5

Closes #332
Add explicit `references()` directive in tests for Rails 4.1 compat

Rails 4.1 no longer scans the SQL conditions string to detect referenced
tables. To fix existing tests, manually specify the referenced tables.
Rails 4.1 compat for association that had `:includes`

The directive on the association no longer seems to have any effect.
Fix `with_scope` test for Rails 4.1
Accept arguments for Relation#count
Forward `count` arguments on Relation for Rails 4.1 compat

Fixes #382, closes #303
Bump mysql2 gem in edge Rails
Use "activerecord-mysql2-adapter" in Rails 3.0 tests
`ActiveRecord::Base.silence` is deprecated
Passing string to `establish_connection` is deprecated
Silence useless minitest warnings

Minitest thinks we're going to use it as a testing frameworks and
complains very loudly that we should require `minitest/autorun` instead.