Update build documents

docs/build.md

@@ -2,10 +2,9 @@
 This page is also available in [Japanese](https://github.com/BoostIO/Boostnote/blob/master/docs/jp/build.md), [Korean](https://github.com/BoostIO/Boostnote/blob/master/docs/ko/build.md), [Russain](https://github.com/BoostIO/Boostnote/blob/master/docs/ru/build.md), [Simplified Chinese](https://github.com/BoostIO/Boostnote/blob/master/docs/zh_CN/build.md), [French](https://github.com/BoostIO/Boostnote/blob/master/docs/fr/build.md) and [German](https://github.com/BoostIO/Boostnote/blob/master/docs/de/build.md).
 
 ## Environments
-* npm: 4.x
-* node: 7.x
 
-You should use `npm v4.x` because `$ grunt pre-build` fails on `v5.x`.
+* npm: 6.x
+* node: 8.x
 
 ## Development
 
@@ -21,17 +20,9 @@ $ yarn
 Build and run.
 
 ```
-$ yarn run dev-start
+$ yarn run dev
 ```
 
-This command runs `yarn run webpack` and `yarn run hot` in parallel. It is the same as running these commands in two terminals.
-
-The `webpack` will watch for code changes and then apply them automatically.
-
-If the following error occurs: `Failed to load resource: net::ERR_CONNECTION_REFUSED`, please reload Boostnote.
-
-![net::ERR_CONNECTION_REFUSED](https://cloud.githubusercontent.com/assets/11307908/24343004/081e66ae-1279-11e7-8d9e-7f478043d835.png)
-
 > ### Notice
 > There are some cases where you have to refresh the app manually.
 > 1. When editing a constructor method of a component
@@ -44,8 +35,6 @@ You can build the program by using `grunt`. However, we don't recommend this bec
 
 So, we've prepared a separate script which just makes an executable file.
 
-This build doesn't work on npm v5.3.0. So you need to use v5.2.0 when you build it.
-
 ```
 grunt pre-build
 ```