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
Tests fail with Rspec 3, so pin at 2.14.x.
Switch to rbx-2 for Travis build.

This should fix the "Requested binary installation but no rubies are available
to download" error in Travis for rbx.
Merge pull request #1075 from mbrictson/pin-rspec-2.14.x

Fix Travis build by pinning Rspec at 2.14.x and switching to rbx-2