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/proftpd.json

@@ -1,21 +1,5 @@
 {
-  "releases": {
-    "1.3.9": {
-      "name": "1.3.9"
-    },
-    "1.3.8": {
-      "name": "1.3.8"
-    },
-    "1.3.7": {
-      "name": "1.3.7"
-    },
-    "1.3.6": {
-      "name": "1.3.6"
-    },
-    "1.3.5": {
-      "name": "1.3.5"
-    }
-  },
+  "releases": {},
   "versions": {
     "1.3.9": {
       "name": "1.3.9",