Make 'build' field in rockspecs mandatory.

Rockspecs must have a 'build' field. If by any chance, it is not needed
(see #379), add this:

````lua
build = { type = "none" }
````

1 files changed, 2 insertions, 1 deletions

diff --git a/src/luarocks/type_check.lua b/src/luarocks/type_check.lua
index 5e3bfb0d..65b4fc15 100644
--- a/src/luarocks/type_check.lua
+++ b/src/luarocks/type_check.lua
@@ -87,7 +87,8 @@ local rockspec_types = {
       copy_directories = {
          _any = string_1,
       },
-      _more = true
+      _more = true,
+      _mandatory = true
    },
    hooks = {
       platforms = {}, -- recursively defined below