fix(package): update browserify to version 15.0.0

Author: greenkeeper[bot]

package.json

@@ -15,7 +15,7 @@
     "@hoodie/client": "^10.0.0",
     "@hoodie/server": "^23.0.0",
     "async": "^2.0.0",
-    "browserify": "^14.0.0",
+    "browserify": "^15.0.0",
     "good": "^7.0.2",
     "good-squeeze": "^5.0.0",
     "h2o2": "^6.0.0",