Fix some style issue

* remove trailing blank (assets.rb)
* remove extra blank line at body end (add_impress_js.rb, document.rb)
* favor if for single line body condition (presentation.rb)
* missing space between { and | in block (add_reveal_js.rb,
  presentation.rb, processor_helpers.rb)
* space missing after comma (add_reveal_js.rb, presentation.rb,
  processor_helpers.rb)
* space missing before } of block (add_reveal_js.rb,
  processors_helpers.rb, assets.rb)
* space missing to the left of { block (add_reveal_js.rb, assets.rb)
* prefer ' (add_reveal_js.rb, add_impress_js.rb)
* use new Hash syntax (presentation.rb)

Thanks to PullReview.com/Ruby Style Guide