Upgrade phantomjs

Fixes https://github.com/badges/shields/issues/645#issuecomment-217222593.

package.json

@@ -19,7 +19,7 @@
     "dot": "~1.0.3",
     "svgo": "~0.5.1",
     "pdfkit": "~0.7.1",
-    "phantomjs": "~1.9.2-6",
+    "phantomjs-prebuilt": "~2.1.7",
     "es6-promise": "~2.1.0",
     "request": "~2.55.0",
     "redis": "~1.0.0",

svg-to-img.js

@@ -1,7 +1,7 @@
 var fs = require('fs');
 var os = require('os');
 var path = require('path');
-var phantom = require('phantomjs');
+var phantom = require('phantomjs-prebuilt');
 var LruCache = require('./lru-cache.js');
 var childProcess = require('child_process');
 var phantomScript = path.join(__dirname, 'phantomjs-svg2png.js');