chore(rockspec) unpin kong version and release new rockspec

Notice how in addition to renaming the rockpec, this makes two changes on the rockspec:

* Changes the `version` field from `0.2.0-0` to `0.2.1`
* Removes `kong` from the `dependencies` table.

The later is done because there is a "circular dependency", which luarocks currently doesn't detect, and this has pernicious consequences when running tests (the CI installs older versions of Kong, and older versions of other gems along that).

Since the change affects only the rockspec, it is not necessary to release a new version of the plugin for this (anyone installing the plugin via luarocks will get the updated luarock).

kong-plugin-zipkin-0.2.0-0.rockspec → kong-plugin-zipkin-0.2.0-1.rockspec

@@ -1,5 +1,5 @@
 package = "kong-plugin-zipkin"
-version = "0.2.0-0"
+version = "0.2.0-1"
 
 source = {
   url = "https://github.com/kong/kong-plugin-zipkin/archive/v0.2.0.zip";
@@ -16,7 +16,6 @@ dependencies = {
   "lua >= 5.1";
   "lua-cjson";
   "lua-resty-http >= 0.11";
-  "kong >= 0.15";
   "opentracing >= 0.0.2";
 }