Exclude lastModifiedMillis in index.json

I noticed that we're including a lastModifiedMillis field in the index.json produced by CollecTor. Here's a local example:

{
  "index_created": "2017-12-14 08:43",
  "build_revision": "56a303e",
  "path": "https://collector.torproject.org",
  "directories": [
    {
      "path": "recent",
      "directories": [
        {
          "path": "bridge-descriptors",
          "directories": [
            {
              "path": "extra-infos",
              "files": [
                {
                  "path": "2017-10-25-18-30-02-extra-infos",
                  "size": 12059081,
                  "last_modified": "2017-10-25 18:30",
                  "lastModifiedMillis": 0     <--------------------
                }
              ]
            },

The reason is that we're using our own Gson instance in CollecTor, and we need to specifically say that we want it to exclude fields without the @Expose annotation. Let's do that. Trivial patch follows.