Create a version file.

I clone and `bundle` and tried to run the spec but do not know why the `Gem.loaded_specs['database_rewinder']` is not working.

Some info:

```bash
$ bundle show database_rewinder
/Users/Mac/dev/database_rewinder
$ rake spec
/Users/Mac/.rvm/rubies/ruby-2.0.0-p247/bin/ruby -S rspec ./spec/active_record_monkey_spec.rb ./spec/database_rewinder_spec.rb
/Users/Mac/dev/database_rewinder/lib/database_rewinder.rb:5:in `<module:DatabaseRewinder>': undefined method `version' for nil:NilClass (NoMethodError)
	from /Users/Mac/dev/database_rewinder/lib/database_rewinder.rb:4:in `<top (required)>'
	from /Users/Mac/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/Mac/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/Mac/dev/database_rewinder/spec/spec_helper.rb:7:in `<top (required)>'
	from /Users/Mac/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/Mac/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/Mac/dev/database_rewinder/spec/active_record_monkey_spec.rb:1:in `<top (required)>'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `load'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `each'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `load_spec_files'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/command_line.rb:22:in `run'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/runner.rb:80:in `run'
	from /Users/Mac/.rvm/gems/ruby-2.0.0-p247@database_rewinder/gems/rspec-core-2.14.7/lib/rspec/core/runner.rb:17:in `block in autorun'
/Users/Mac/.rvm/rubies/ruby-2.0.0-p247/bin/ruby -S rspec ./spec/active_record_monkey_spec.rb ./spec/database_rewinder_spec.rb failed
```

So I propose create a `version.rb` file and load it like so in this Pull Request. :smiley: