Remove invalid releases automatically (#466)

Automatically remove empty releases or releases which are released in the future, regardless of the product.

Also refactored a bit releasedata.py to use better names and clarify types.

releases/mongodb.json

@@ -103,33 +103,6 @@
       "name": "3.0",
       "releaseDate": "2015-03-03",
       "eol": "2018-02-28"
-    },
-    "2.6": {
-      "name": "2.6"
-    },
-    "2.4": {
-      "name": "2.4"
-    },
-    "2.2": {
-      "name": "2.2"
-    },
-    "2.0": {
-      "name": "2.0"
-    },
-    "1.8": {
-      "name": "1.8"
-    },
-    "1.6": {
-      "name": "1.6"
-    },
-    "1.4": {
-      "name": "1.4"
-    },
-    "1.2": {
-      "name": "1.2"
-    },
-    "1.0": {
-      "name": "1.0"
     }
   },
   "versions": {