Update GitHub Octocat Mark PNG with new branding, adjust position

Replaced classic Octocat Mark with new "small" variant Octocat
Mark for GitHub slide deck theme. Logo now appears in upper right
for better alignment and avoids overlap of any slide deck content.
Reverse merge to update TravisCI configuration
Merge pull request #212 from github/octocat-mark-update

Update GitHub Octocat Mark PNG with new branding