[logs] fixing small bug in carbonblack log schema

[ryandeivert]

to @jacknagz
cc @airbnb/streamalert-maintainers

bug fix

• Fixing small bug in carbonblack:watchlist.hit.binary declared schema. It turns out the timestamp stored at the root of the log is a float, but the timestamp stored within each entry of the docs list (inside the 'envelope') is a string value (ie: 2017-01-01T20:55:57.828Z).
• Example:

{
  "cb_server": "cbserver",
  "cb_version": "6.1.0.170309.1140",
  "docs": [
    {
      "cb_version": "524",
      "comments": "comment",
      "company_name": "Foobar Corp",
      "copied_mod_len": "284904",
      "digsig_issuer": "Foobar Inc",
      "digsig_result": "Untrusted",
      "digsig_result_code": "123123123123",
      "digsig_subject": "Internal",
      "endpoint": [
        "1LAKDJ-F93J1"
      ],
      "event_partition_id": [
        "111111111111"
      ],
      "facet_id": "0",
      "file_desc": "Foobar Tech",
      "file_version": "1.0.0",
      "group": [
        "FooBar"
      ],
      "host_count": 1,
      "internal_name": "foobar-2-0",
      "is_64bit": "false",
      "is_executable_image": "false",
      "last_seen": "2017-03-01T00:00:50.635Z",
      "legal_copyright": "Copyright (C) 2011 Foobar Corporation",
      "md5": "0000000000000000000000000000",
      "observed_filename": [
        "c:\\program files (x86)\\foobar\\lolevildawg.dll"
      ],
      "orig_mod_len": "222222",
      "original_filename": "asdfasdfasdfasdf",
      "os_type": "Windows",
      "product_name": "Foobar Technology",
      "product_version": "1.0.0",
      "server_added_timestamp": "2017-01-00T20:55:57.828Z",
      "signed": "Untrusted Root",
      "timestamp": "2017-01-01T20:55:57.828Z"
    }
  ],
  "highlights_by_doc": {},
  "server_name": "foobar.my.carbonblack.io",
  "timestamp": "1491264681.15",
  "type": "watchlist.hit.binary",
  "watchlist_id": "593",
  "watchlist_name": "tmp-delete-me-bad-signature"
}

[coveralls]

Coverage remained the same at 79.307% when pulling 8ee5953 on ryandeivert-carbonblack-watchlist.hit.binary-log-fix into eaa76bb on master.