Reword the 'add gems' slide to make it clearer what to add

The sqlite3 gem should be already there. We want to add `gem
'carrierwave'` below it.
Make it clear what `... if @idea.picture.present?` does
Fix a mistake on the 'root redirect' slide

We had to manually go to `/ideas`, not `/index`.
Fix small typo in the 'static page' slide
Remove branch name from the git prompt before `git init`
Merge pull request #1 from stormbreakerbg/guide-fixes

Some small fixes for things I noticed