correct the branch name in rockspec (#90)

diff --git a/rockspec/skywalking-nginx-lua-0.6.0-0.rockspec b/rockspec/skywalking-nginx-lua-0.6.0-0.rockspec
index 6032716..6e06236 100644
--- a/rockspec/skywalking-nginx-lua-0.6.0-0.rockspec
+++ b/rockspec/skywalking-nginx-lua-0.6.0-0.rockspec
@@ -2,7 +2,7 @@
 version = "0.6.0-0"
 source = {
    url = "git://github.com/apache/skywalking-nginx-lua",
-   branch = "0.6.0",
+   branch = "v0.6.0",
 }
 
 description = {