Treat issue dates as datetime type

Fixes singer-io#56.

Author: Sam Minnée

tap_github/issues.json

@@ -57,7 +57,8 @@
       "type": [
         "null",
         "string"
-      ]
+      ],
+      "format": "date-time"
     },
     "labels_url": {
       "type": [
@@ -82,7 +83,8 @@
       "type": [
         "null",
         "string"
-      ]
+      ],
+      "format": "date-time"
     },
     "html_url": {
       "type": [
@@ -162,7 +164,8 @@
       "type": [
         "null",
         "string"
-      ]
+      ],
+      "format": "date-time"
     },
     "_sdc_repository": {
       "type": ["string"]