Switch to high-quality SVG badges

This is so that the badges don't look like a blurry mess on retina screens.

Note that this also fixes the Travis status, which was previously reporting green. It is now correctly showing "error", matching the status shown here: https://travis-ci.org/capistrano/capistrano
Merge pull request #1074 from mbrictson/patch-1

Switch to high-quality SVG badges