btc: also usuable with cjson

contrib/README.md

@@ -64,7 +64,10 @@ Supported platforms: Linux (required tools: `sysfs`)
 
 Provides current Bitcoin price in any currency by code (
 https://en.wikipedia.org/wiki/ISO_4217).
-Supported platforms: Linux, FreeBSD (required tools: `curl`)
+Requires `curl` and one of the following json libraries:
+ - [lua-cjson](https://github.com/mpx/lua-cjson/))
+ - [luajson](https://github.com/harningt/luajson/)
+Supported platforms: Linux, FreeBSD
 
 - Arguments:
   * takes currency code, i.e. `"usd"`, `"rub"` and other. `"usd"` by

contrib/btc_all.lua

@@ -8,7 +8,12 @@ local setmetatable = setmetatable
 local pcall = pcall
 local helpers = require("vicious.helpers")
 local spawn = require("awful.spawn")
-local json = require("json")
+
+local success, json = pcall(require, "cjson")
+if not success then
+  json = require("json")
+end
+
 local string = {
     sub = string.sub,
     upper = string.upper,