Replace PNG icons with shields.io-style SVG icons

Fixes https://issues.jenkins-ci.org/browse/JENKINS-23062 and
https://issues.jenkins-ci.org/browse/JENKINS-21112

src/main/java/org/jenkinsci/plugins/badge/ImageResolver.java

@@ -33,11 +33,11 @@ public class ImageResolver {
 
     public ImageResolver() throws IOException{
         images = new StatusImage[] {
-                new StatusImage("failure.png"),
-                new StatusImage("unstable.png"),
-                new StatusImage("success.png"),
-                new StatusImage("running.png"),
-                new StatusImage("unknown.png")
+                new StatusImage("build-failure-red.svg"),
+                new StatusImage("build-unstable-yellow.svg"),
+                new StatusImage("build-passing-brightgreen.svg"),
+                new StatusImage("build-running-blue.svg"),
+                new StatusImage("build-unknown-lightgrey.svg")
         };
     }
 

src/main/java/org/jenkinsci/plugins/badge/StatusImage.java

@@ -65,7 +65,7 @@ public void generateResponse(StaplerRequest req, StaplerResponse rsp, Object nod
         rsp.setHeader("ETag",etag);
         rsp.setHeader("Expires","Fri, 01 Jan 1984 00:00:00 GMT");
         rsp.setHeader("Cache-Control", "no-cache, private");
-        rsp.setHeader("Content-Type", "image/png");
+        rsp.setHeader("Content-Type", "image/svg+xml;charset=utf-8");
         rsp.setHeader("Content-Length", length);
         rsp.getOutputStream().write(payload);
     }