CI: Use CircleCI build # for Sauce screenshot build name too

Same format as Sauce Labs unit test build name
diff --git a/script/screenshots.js b/script/screenshots.js
index a28f035..b71f1b1 100644
--- a/script/screenshots.js
+++ b/script/screenshots.js
@@ -19,6 +19,15 @@
 var baseDir = process.env['CIRCLE_ARTIFACTS'] || '/tmp';
 var url = process.argv[2];
 var allImgsDir = baseDir+'/imgs';
+var build_name = 'CircleCI build #' + process.env.CIRCLE_BUILD_NUM;
+if (process.env.CIRCLE_PR_NUMBER) {
+  build_name += ': PR #' + process.env.CIRCLE_PR_NUMBER;
+  if (process.env.CIRCLE_BRANCH) build_name += ' (' + process.env.CIRCLE_BRANCH + ')';
+} else {
+  build_name += ': ' + process.env.CIRCLE_BRANCH;
+}
+build_name += ' @ ' + process.env.CIRCLE_SHA1.slice(0, 7);
+
 fs.mkdirSync(allImgsDir);
 
 var browserVersions = [
@@ -78,6 +87,7 @@
 
 browserVersions.forEach(function(obj) {
   var cfg = obj.version;
+  cfg.build = build_name;
   var browserDriver = wd.remote('ondemand.saucelabs.com', 80, username, accessKey);
   // The following is in the style of
   // https://github.com/admc/wd/blob/62f2b0060d36a402de5634477b26a5ed4c051967/examples/async/chrome.js#L25-L40