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"
  },