[index.html.erb] Rewrote IndexHtml from an erb approach

Previously, it was unpacking whatever reveal.js came with, and
transforming that into the presentation of the user.

This came from the mindset where I thought I'd be upgraded more
frequently, and I wanted to "transform whatever reveal.js' index" looked
like into the final result.

No more-- with this approach we'll explicitly maintain an index.html.erb
that may need to be updated each time the embedded reveal.js is
embedded. But-- the thinking is that this is just fine because it
doesn't happen very often and reveal.js isn't changing very much.
[index.html.erb] Reformatted index.html.erb

* Should only be whitespace changes.
* 2 space indent
[index.html.erb] Removed files no longer necessary when taking .erb approach
[index.html.erb] Converted CreateIndexHtml to CreationTask
Simplified index.html creation with erb

Previously, the paradigm was that index.html would start with whatever
reveal.js had, and we'd change it for the user.

No longer-- now we've got an official template for index.html.. and it
matches reveal.js today-- but it may need to be updated in the future.

However-- I think it'll be easy to update-- and it makes the processing
a whole bunch easier.

Merge branch 'index.html.erb' into upcoming

* index.html.erb:
  [index.html.erb] Converted CreateIndexHtml to CreationTask
  [index.html.erb] Removed files no longer necessary when taking .erb approach
  [index.html.erb] Reformatted index.html.erb
  [index.html.erb] Rewrote IndexHtml from an erb approach
[rubocop] Modified code to please Rubocop

I was locked on an older version (in my Gemfile.lock) and didn't notice
until I pushed up to travis..