Add rdiscount and RedCloth to Gemfile

`bin/build` fails without them.

Also s/:rubygems/https://rubygems.org to prevent deprecation warning when bundler fires.
Merge pull request #80 from benbalter/gemfile

Add rdiscount and RedCloth to Gemfile
Subject/verb agreement (as I read it)

“GitHub” is consistently referred to in the 3rd person, and I thought the sentence reads better when that is maintained. Also, switched straight quotations marks to typographic ones, because why settle for less?
Removing duplicate h1 with title

The bare.html layout file already outputs the page.title as an h1, so blog posts currently get duplicate titles (see [Refactoring](http://teach.github.com/articles/Refactoring/) or [Materials launched](http://teach.github.com/articles/Materials-Launched/))
Merge pull request #85 from acusti/patch-2

Removing duplicate h1 with title
Merge pull request #84 from acusti/patch-1

Subject/verb agreement (as I read it)
Update h3 style and alignment
Merge pull request #86 from github/update-h3-styles

Update h3 style and alignment