Use SVG for status badges in README

http://shields.io/ provides many standardized badges for OSS. The PNG versions provided by Shields are similar to the images currently used, but the SVG versions added here are more legible on high pixel density displays.

README.md

@@ -1,4 +1,4 @@
-# PaperTrail [![Build Status](https://secure.travis-ci.org/airblade/paper_trail.png?branch=master)](http://travis-ci.org/airblade/paper_trail) [![Dependency Status](https://gemnasium.com/airblade/paper_trail.png)](https://gemnasium.com/airblade/paper_trail)
+# PaperTrail [![Build Status](https://img.shields.io/travis/airblade/paper_trail/master.svg)](https://travis-ci.org/airblade/paper_trail) [![Dependency Status](https://img.shields.io/gemnasium/airblade/paper_trail.svg)](https://gemnasium.com/airblade/paper_trail)
 
 PaperTrail lets you track changes to your models' data.  It's good for auditing or versioning.  You can see how a model looked at any stage in its lifecycle, revert it to any version, and even undelete it after it's been destroyed.