Update to the installation instructions

It was really unclear that the new versions of Elixir require version 17 of erlang and not the version R16 that was fine with erlier versions of elixir (0.11.2 for example). So someone who was upgrading could totally miss it.
Create an element id for  #stable, in addition to the version number.
Fix warnings generated by layout:nil, and pygments:
nodejs is required
Merge pull request #383 from eksperimental/requirements

nodejs is required
Merge pull request #381 from eksperimental/stable_id

Create an element id for  #stable, in addition to the version number.
Merge pull request #382 from eksperimental/warnings

Fix warnings generated by layout:nil, and pygments:
Be more explicit on the node dependency
Merge pull request #380 from aseeon/master

Update to the installation instructions
Add a brief mention of Erlang version instead
Update learning-resources.html
Merge pull request #384 from mosic/patch-1

Update Programming Elixir book cover (it's out of beta)
Bring back syntax highlight
Update 1.markdown
Unfortunately Fedora packages are out of date
Use HashDict.has_key? instead of HashDict.get

Because readability and consistency and what happens when the value at the key is FALSE
Merge pull request #391 from L8D/patch-1

Use HashDict.has_key? instead of HashDict.get
Repharse explanation of linked lists
Merge pull request #393 from thmzlt/patch-1

Repharse explanation of linked lists
Talk about char lists early on