ci: run lua test cases with luajit (#94)

* ci: run lua test cases with luajit

since lua-cjson 2.1.0-1 may fail to install, ref
https://github.com/openresty/lua-cjson/issues/21, so use lua-cjson2,
ref: https://github.com/leafo/lapis/issues/539#issuecomment-319393395
diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index e551ac7..c33c63c 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -47,19 +47,18 @@
         shell: bash
         run: |
           sudo apt-get update -y
-          sudo apt-get install -y lua5.3 lua5.3-dev
-          sudo ln /usr/bin/lua5.3 /usr/bin/lua
+          sudo apt-get install -y lua5.1 lua5.1-dev
       - name: 'Install LuaRocks'
         run: |
-          wget https://luarocks.org/releases/luarocks-3.3.0.tar.gz
-          tar zxpf luarocks-3.3.0.tar.gz
-          cd luarocks-3.3.0
+          wget https://luarocks.org/releases/luarocks-3.8.0.tar.gz
+          tar zxpf luarocks-3.8.0.tar.gz
+          cd luarocks-3.8.0
           ./configure && make && sudo make install
           cd ..
       - name: 'Install Dependencies'
         run: |
           sudo luarocks install luaunit
-          sudo luarocks install lua-cjson 2.1.0-1
+          sudo luarocks install lua-cjson2
           sudo luarocks install lua-resty-jit-uuid
       - name: "Install OpenResty"
         run: |
@@ -77,11 +76,11 @@
       - name: 'Run Lua Tests'
         run: |
           cd lib
-          lua skywalking/util_test.lua
-          lua skywalking/span_test.lua
-          lua skywalking/tracing_context_test.lua
-          lua skywalking/segment_ref_test.lua
-          lua skywalking/correlation_context_test.lua
+          /usr/local/openresty-debug/luajit/bin/luajit skywalking/util_test.lua
+          /usr/local/openresty-debug/luajit/bin/luajit skywalking/span_test.lua
+          /usr/local/openresty-debug/luajit/bin/luajit skywalking/tracing_context_test.lua
+          /usr/local/openresty-debug/luajit/bin/luajit skywalking/segment_ref_test.lua
+          /usr/local/openresty-debug/luajit/bin/luajit skywalking/correlation_context_test.lua
           cd ..
       - name: 'Run Nginx Lua Tests'
         run: |
diff --git a/README.md b/README.md
index 1f09aac..026cf41 100644
--- a/README.md
+++ b/README.md
@@ -117,7 +117,7 @@
 - LuaRocks
 
 The following libs are required in runtime or test cases, please use `LuaRocks` to install them.
-- lua-cjson. NOTICE, some platforms such as MacOS 10.15 may have issue with the latest release of this lib, consider to install an old release.(`luarocks install lua-cjson 2.1.0-1`)
+- lua-cjson2. NOTICE, lua-cjson2 is to fix the bug in lua-cjson 2.1.0-1. ref to: https://github.com/apache/skywalking-nginx-lua/pull/94
 - luaunit
 - lua-resty-jit-uuid