Please add parameter "--allow-hash-href" to call htmlproofer in Continuous integration with Travis CI

I got stuck in Step 10: Merge the unit test of Continuous integration with Travis CI because testlinks couldn’t pass. By checking travis CI log, I figured out it’s related to htmlproofer:

[...]
Checking 6 external links...
Ran on 4 files!
- ./_site/about/index.html
  *  linking to internal hash # that does not exist (line 1)
     <a href="#" id="menu-button" class="site-button">[...]
- ./_site/docs/01_getting_ready_for_class/index.html
  *  linking to internal hash # that does not exist (line 1)
     <a href="#" id="menu-button" class="site-button">[...]
- ./_site/docs/02_getting_started/index.html
  *  linking to internal hash # that does not exist (line 1)
     <a href="#" id="menu-button" class="site-button">[...]
- ./_site/index.html
  *  External link https://help.github.com/setupgit failed: 404 No error
  *  linking to internal hash # that does not exist (line 1)
     <a href="#" id="menu-button" class="site-button">[...]
htmlproofer 3.15.3 | Error:  HTML-Proofer found 5 failures!
The command "script/testlinks" exited with 1.

I passed the test by modifying script/testlinks. I appended --allow-hash-href to bundle exec htmlproofer ./_site --check-html.