remove @ version for extract-text-webpack-plugin (close #747)

Author: yyx990803

docs/en/configurations/extract-css.md

@@ -5,7 +5,7 @@ Example config to extract all the processed CSS in all Vue components into a sin
 ### Webpack 2.x
 
 ``` bash
-npm install extract-text-webpack-plugin@2.x --save-dev
+npm install extract-text-webpack-plugin --save-dev
 ```
 
 ``` js

docs/ja/configurations/extract-css.md

@@ -5,7 +5,7 @@
 ### Webpack 2.x
 
 ``` bash
-npm install extract-text-webpack-plugin@2.x --save-dev
+npm install extract-text-webpack-plugin --save-dev
 ```
 
 ``` js

docs/kr/configurations/extract-css.md

@@ -38,7 +38,7 @@ module.exports = {
 ### Webpack 2.x (^2.1.0-beta.25)
 
 ``` bash
-npm install extract-text-webpack-plugin@2.x --save-dev
+npm install extract-text-webpack-plugin --save-dev
 ```
 
 ``` js

docs/pt_BR/configurations/extract-css.md

@@ -5,7 +5,7 @@ Exemplo de configuração para extrair todo CSS processado de todos componentes
 ### Webpack 2.x
 
 ```bash
-npm install extract-text-webpack-plugin@2.x --save-dev
+npm install extract-text-webpack-plugin --save-dev
 ```
 
 ```js
@@ -67,4 +67,4 @@ module.exports = {
     new ExtractTextPlugin("style.css")
   ]
 }
-```
+```

docs/ru/configurations/extract-css.md

@@ -5,7 +5,7 @@
 ### Webpack 2.x
 
 ``` bash
-npm install extract-text-webpack-plugin@2.x --save-dev
+npm install extract-text-webpack-plugin --save-dev
 ```
 
 ``` js

docs/zh-cn/configurations/extract-css.md

@@ -5,7 +5,7 @@
 ### Webpack 2.x
 
 ``` bash
-npm install extract-text-webpack-plugin@2.x --save-dev
+npm install extract-text-webpack-plugin --save-dev
 ```
 
 ``` js