update vega to 2.5.1

Author: kanitw

package.json

@@ -68,7 +68,7 @@
     "typescript": "^1.8.2",
     "typescript-json-schema": "^0.0.8",
     "uglify-js": "^2.6.2",
-    "vega": "^2.5.0",
+    "vega": "^2.5.1",
     "vega-datasets": "vega/vega-datasets#gh-pages",
     "watchify": "^3.7.0",
     "z-schema": "^3.16.1"