Migrate to Mattermost 5.2 new plugin system

Cf. https://forum.mattermost.org/t/plugin-system-upgrade-in-mattermost-5-2/5498

Gopkg.toml

@@ -31,7 +31,7 @@
 
 [[constraint]]
   name = "github.com/mattermost/mattermost-server"
-  version = "4.10.0"
+  version = "5.2.0"
 
 [[constraint]]
   name = "github.com/stretchr/testify"

README.md

@@ -2,7 +2,9 @@
 This Mattermost plugin adds a `/roll` slash command to roll all kinds of virtual dice.
 
 ## Requirements
-- Mattermost 4.6 (to allow plugins to create slash commands) 
+- for Mattermost 5.2 or higher: use the latest v2.x.x release
+- for Mattermost 4.6 to 5.1: use the latest v1.x.x release
+- for Mattermost below: unsupported versions (plugins can't create slash commands)
 
 ## Installation and configuration
 1. Go to the [Releases page](https://github.com/moussetc/mattermost-plugin-dice-roller/releases) and download the package for your OS and architecture.
@@ -44,4 +46,4 @@ If you want to create a fully bundled plugin that will run on a local server, yo
 - Generate roll results as accurate image (number of dice, number of faces...)
 
 ## Credits
-- This project uses a dice icon provided by [openclipart](https://openclipart.org/detail/94501/twentysided-dice) under the [Creative Commons Zero 1.0 Public Domain License](https://creativecommons.org/publicdomain/zero/1.0/).
+- This project uses a dice icon provided by [openclipart](https://openclipart.org/detail/94501/twentysided-dice) under the [Creative Commons Zero 1.0 Public Domain License](https://creativecommons.org/publicdomain/zero/1.0/).