Added gemfiles docu & still have trouble setting up specs =(

Hi, I've added some info on how to set up the test suite to the README.

I still have trouble running the specs, even with a plain `rspec spec` (using both Ruby 1.9.3-p429 and 2.0.0-p195).

First of all i run into a couple of these errors:

```
/Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/activemodel-4.0.0/lib/active_model/deprecated_mass_assignment_security.rb:17:in `attr_accessible': `attr_accessible` is extracted out of Rails into a gem. Please use new recommended protection model for params(strong_parameters) or add `protected_attributes` to your Gemfile to use old one. (RuntimeError)
```

After I commented out all attr_accessible calls, I still get this one (which is also the first error I get when running `rake`):

```
/Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/active_record/truncation.rb:123:in `db_version': undefined local variable or method `postgresql_version' for #<ActiveRecord::ConnectionAdapters::SQLite3Adapter:0x007f909c2da580> (NameError)
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/active_record/truncation.rb:131:in `restart_identity'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/active_record/truncation.rb:140:in `truncate_tables'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/active_record/truncation.rb:234:in `block in clean'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract_adapter.rb:307:in `disable_referential_integrity'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/active_record/truncation.rb:230:in `clean'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/base.rb:40:in `clean_with'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/configuration.rb:85:in `block in clean_with'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/configuration.rb:85:in `each'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/database_cleaner-1.1.1/lib/database_cleaner/configuration.rb:85:in `clean_with'
	from /Users/paul/github/money-rails/spec/support/database_cleaner.rb:6:in `block (2 levels) in <top (required)>'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/hooks.rb:21:in `instance_eval'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/hooks.rb:21:in `run'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/hooks.rb:85:in `block in run'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/hooks.rb:85:in `each'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/hooks.rb:85:in `run'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/hooks.rb:446:in `run_hook'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/command_line.rb:27:in `block in run'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/reporter.rb:58:in `report'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/command_line.rb:25:in `run'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/runner.rb:80:in `run'
	from /Users/paul/.rbenv/versions/1.9.3-p429/lib/ruby/gems/1.9.1/gems/rspec-core-2.14.5/lib/rspec/core/runner.rb:17:in `block in autorun'
```