fix grammar so that it works with lpeg 0.12

fix proposed by Roberto Ierusalimschy on lua-l

src/cosmo/grammar.lua

@@ -37,12 +37,10 @@ local longstring1 = lpeg.P{
   longstring = lpeg.P"[[" * (lpeg.V"longstring" + (lpeg.P(1) - lpeg.P"]]"))^0 * lpeg.P"]]"
 }
 
-local longstring2 = lpeg.P(function (s, i)
-  local l = lpeg.match(start, s, i)
-  if not l then return nil end
-  local p = lpeg.P("]" .. string.rep("=", l - i - 2) .. "]")
-  p = (1 - p)^0 * p
-  return lpeg.match(p, s, l)
+local longstring2 = lpeg.Cmt(start, function (s, i, start)
+  local p = string.gsub(start, "%[", "]")
+  local _, e = string.find(s, p, i)
+  return (e and e + 1)
 end)
 
 local longstring = #("[" * lpeg.S"[=") * (longstring1 + longstring2)