Remove @next instructions from documentation

Also adjust the script that sets release id in `_config.yml` to no longer make
any assumptions about pre-releases

docs/_config.yml

@@ -6,7 +6,6 @@ url: 'http://sinonjs.org'
 github_username: sinonjs
 sinon:
   current_release: v2.0.0
-  next_release: v2.0.0-pre.6
 markdown: kramdown
 kramdown:
   input: GFM

docs/index.md

@@ -4,7 +4,6 @@ title: Sinon.JS
 ---
 
 {% assign current_release = site.sinon.current_release %}
-{% assign next_release = site.sinon.next_release %}
 
 # Sinon.JS
 
@@ -25,12 +24,6 @@ To install the current release (`{{current_release}}`) of Sinon:
 npm install sinon
 ```
 
-If you're interested in evaluating the next release of Sinon (`{{next_release}}`)
-
-```shell
-npm install sinon@next
-```
-
 If you (for some reason) really want the old version of Sinon, head over to the [legacy docs](http://legacy.sinonjs.org/) and do:
 
 ```shell

scripts/set-release-id-in-config-yml.js

@@ -5,12 +5,8 @@ var UTF8 = "utf8";
 var fs = require("fs");
 var yaml = require("js-yaml");
 var releaseId = "v" + require("../package.json").version;
-
-var isNextRelease = releaseId.includes("pre");
-var releaseKey = isNextRelease ? "next_release" : "current_release";
-
 var config = yaml.safeLoad(fs.readFileSync(configYmlPath, UTF8));
 
-config.sinon[releaseKey] = releaseId;
+config.sinon.current_release = releaseId;  // eslint-disable-line camelcase
 
 fs.writeFileSync(configYmlPath, yaml.safeDump(config), UTF8);