Delete unknown constant HTML2Slim::B

````
<module:HTML2Slim>': uninitialized constant HTML2Slim::B (NameError)
	from lib/html2slim/converter.rb:3:in `<top (required)>'
````
Merge pull request #17 from bash0C7/fix_uninitialized_constant_HTML2Slim_B

Fix test failed by '`<module:HTML2Slim>': uninitialized constant HTML2Slim::B (NameError)'