fix(package): update base62 to version 1.2.5

Closes #4362

package.json

@@ -151,7 +151,7 @@
   "homepage": "https://github.com/mozillla/addons-frontend#readme",
   "dependencies": {
     "babel-polyfill": "6.26.0",
-    "base62": "1.2.1",
+    "base62": "1.2.5",
     "base64url": "2.0.0",
     "better-npm-run": "0.1.0",
     "bunyan": "1.8.12",