WIP Rails 4

* [x] add support for Rails 4
* [x] remove gemfiles/*.lock files

I still get 1 failing test, but I see it in master as well

```shell
$ be rake appraisal:rails4.0.mysql2 test
>> BUNDLE_GEMFILE=/Users/lukkry/projects/zendesk/active_record_host_pool/gemfiles/rails4.0.mysql2.gemfile bundle exec /Users/lukkry/.rbenv/versions/2.0.0-p353/bin/rake test
/Users/lukkry/.rbenv/versions/2.0.0-p353/bin/ruby -I"lib:lib" -I"/Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rake-10.1.1/lib" "/Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rake-10.1.1/lib/rake/rake_test_loader.rb" "test/test_arhp.rb"
Run options:

# Running tests:

[11/11] ActiveRecordHostPoolTest#test_underlying_assumption_about_test_db = 0.56 s
  1) Error:
ActiveRecordHostPoolTest#test_underlying_assumption_about_test_db:
ActiveRecord::StatementInvalid: Mysql2::Error: Query execution was interrupted: KILL 355
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:287:in `query'
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:287:in `block in execute'
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_adapter.rb:435:in `block in log'
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activesupport-4.0.2/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_adapter.rb:430:in `log'
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:287:in `execute'
    /Users/lukkry/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/mysql2_adapter.rb:222:in `execute'
    /Users/lukkry/projects/zendesk/active_record_host_pool/lib/active_record_host_pool/connection_adapter_mixin.rb:25:in `execute_with_switching'
    /Users/lukkry/projects/zendesk/active_record_host_pool/test/test_arhp.rb:100:in `test_underlying_assumption_about_test_db'
```