Change .busted lua to nlua with no absolute path

For reasons outlined in #19

Author: mfussenegger

.busted

@@ -2,7 +2,7 @@ return {
   _all = {
     coverage = false,
     lpath = "lua/?.lua;lua/?/init.lua",
-    lua = "~/.luarocks/bin/nlua",
+    lua = "nlua",
   },
   default = {
     verbose = true