Fix "uninitialized constant Configspec::Helper::Dockerfile (NameError)"

I have tried configspec-init with Dockerfile, it didn't work. This will fix for it.

```
/Users/gabu/.rbenv/versions/2.0.0-p353/bin/ruby -S rspec
/Users/gabu/git/dev/configspec/hello/spec/spec_helper.rb:3:in `<top (required)>': uninitialized constant Configspec::Helper::Dockerfile (NameError)
	from /Users/gabu/git/dev/configspec/hello/spec/001_httpd_spec.rb:1:in `require'
	from /Users/gabu/git/dev/configspec/hello/spec/001_httpd_spec.rb:1:in `<top (required)>'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `each'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load_spec_files'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/command_line.rb:22:in `run'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:80:in `run'
	from /Users/gabu/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:17:in `block in autorun'
/Users/gabu/.rbenv/versions/2.0.0-p353/bin/ruby -S rspec failed
```