travis: failed to run test case on osx now. (#474)

* travis: enabled v2 protocal for etcd under osx.

.travis/osx_runner.sh

@@ -43,7 +43,7 @@ script() {
     export_or_prefix
     export PATH=$OPENRESTY_PREFIX/nginx/sbin:$OPENRESTY_PREFIX/luajit/bin:$OPENRESTY_PREFIX/bin:$PATH
 
-    brew services start etcd
+    etcd --enable-v2=true &
     sleep 1
 
     luarocks install luacheck

README.md

@@ -81,6 +81,8 @@ You now have two ways to install APISIX: if you are using CentOS 7, it is recomm
 
 We will add support for Docker and more OS shortly.
 
+*NOTE*: APISIX currently only supports the v2 protocol storage to etcd, but the latest version of etcd (starting with 3.4) has turned off the v2 protocol by default. You need to add `--enable-v2=true` to the startup parameter to enable the v2 protocol. The development of the v3 protocol supporting etcd has begun and will soon be available.
+
 ### Install from RPM for CentOS 7
 
 ```shell

README_CN.md

@@ -76,6 +76,9 @@ APISIX 在以下操作系统中做过安装和运行测试:
 
 现在有两种方式来安装: 如果你是 CentOS 7 的系统，推荐使用 RPM 包安装；其他的系统推荐使用 Luarocks 安装。
 
+*NOTE*: APISIX 目前仅支持 etcd 的 v2 协议存储，但最新版的 etcd (3.4 开始）已经默认关闭 v2 协议。
+需要在启动参数中添加 `--enable-v2=true`，才能启用 v2 协议。支持 etcd 的 v3 协议开发工作已经开始，很快就能与大家见面。
+
 ### 通过 RPM 包安装（CentOS 7）
 
 ```shell