Remove Yarn references in favor of npm 5

Author: Hyunje Jun

README.md

@@ -8,11 +8,10 @@ Node.js SDK for LINE Messaging API
 
 ### Install
 
-Using [npm](https://www.npmjs.com/) or [Yarn](https://yarnpkg.com/):
+Using [npm](https://www.npmjs.com/):
 
 ``` bash
 $ npm install @line/bot-sdk
-$ yarn add @line/bot-sdk
 ```
 
 ### Documentation

docs/pages/getting-started/install.md

@@ -1,10 +1,9 @@
 # Install
 
-Please install via [npm](https://www.npmjs.com/) or [Yarn](https://yarnpkg.com/).
+Please install via [npm](https://www.npmjs.com/).
 
 ```bash
 $ npm install @line/bot-sdk
-$ yarn add @line/bot-sdk
 ```
 
 You can build from source. Please clone the repository and run the following

docs/pages/getting-started/requirements.md

@@ -1,9 +1,7 @@
 # Requirements
 
-* **Node.js** >= 4
+* **Node.js** >= 4, preferably >=6
   * It uses ES2015.
-* Package manager for **Node.js**, such as [**npm**](https://www.npmjs.com/) or
-  [**Yarn**](https://yarnpkg.com/)
+* [**npm**](https://www.npmjs.com/), preferably >=5
 
-Other dependencies are installed via the package manager, and do not need to be
-pre-installed.
+Other dependencies are installed via npm, and do not need to be pre-installed.

examples/echo-bot/README.md

@@ -8,7 +8,6 @@ Install deps:
 
 ```bash
 $ npm install
-$ yarn # alternatively
 ```
 
 Configuration: