meta: update dates for 4.x EOL and 6.x maintenance

I've updated the schedule.json and the README.md

Not sure the best way to update schedule.ical, how do we create it?

There is a part of me that thinks we should remove it and put the dates
on the official calendar. I'll open another PR for that

Closes: #264

README.md

@@ -6,9 +6,9 @@
 |   :--:  |    :---:      |   :---:  |       :---:      |       :---:       |      :---:      |
 |  v0.10.x  |**End-of-Life**|    -     |        -         |    2015-10-01     |   2016-10-31    |
 |  v0.12.x  |**End-of-Life**|    -     |        -         |    2016-04-01     |   2016-12-31    |
-| [4.x][] |**Maintenance**| [Argon][]|    2015-10-01    |    2017-04-01     |   April 2018    |
+| [4.x][] |**Maintenance**| [Argon][]|    2015-10-01    |    2017-04-01     |   2018-04-04    |
 |  5.x    |No LTS         |          |                  |                   |                 |
-| [6.x][] |**Active**     | [Boron][]|    2016-10-18    |    April 2018     |   April 2019    |
+| [6.x][] |**Active**     | [Boron][]|    2016-10-18    |    2018-04-04    |   April 2019    |
 |  7.x    |No LTS         |          |                  |                   |                 |
 |  8.x    |**Pending**    | Carbon  |    2017-10-31  |    April 2019     |   December 2019<sup>2</sup> |
 |  9.x    |No LTS         |          |                  |                   |                 |

schedule.json

@@ -11,7 +11,7 @@
     "start": "2015-09-08",
     "lts": "2015-10-12",
     "maintenance": "2017-04-01",
-    "end": "2018-04-01",
+    "end": "2018-04-04",
     "codename": "Argon"
   },
   "v5": {
@@ -22,7 +22,7 @@
   "v6": {
     "start": "2016-04-26",
     "lts": "2016-10-18",
-    "maintenance": "2018-04-01",
+    "maintenance": "2018-04-04",
     "end": "2019-04-01",
     "codename": "Boron"
   },
@@ -39,7 +39,7 @@
     "codename": "Carbon"
   },
   "v9": {
-    "start": "2017-10-01",
+    "start": "2017-10-31",
     "maintenance": "2018-04-01",
     "end": "2018-06-30"
   },