support screen readers

[fix #171] remove "clip-path" attribute

Fix class mismatch in formatting lesson

The 'input' (raw) block showed the `.source` class identifier, but the rendered output used `.language-python`.

Ensure  Ruby is updated to work with Jekyll on OSX

If you have an out dated Ruby version. 
They may fail to work with jekyll

fix link to setup instructions

update maintenance process

remove episode length

indent HTML

update info on formative assessments

Add HTML indenting.

jekyll version

Add note about version of Jekyll needed per https://github.com/swcarpentry/styles/issues/195#issuecomment-351056899.

fixed broken link on landing page

Fix link to setup page in index

Add Library Carpentry website to _config.yml

Close #150

Close #152.

Per _config.yml in workshop-template, adding Library Carpentry website info for appropriate linking in _includes/lesson_footer.html

The link to setup is broken. This pull request suggest to link to the existing setup page (`{{ page.root}}/setup.html`), but other options are 
 to change the permalink on Setup to `{{ site.url }}/setup/`

Syntax highlighting example