Define command intallation the right way.

metalua-0.7.3-1.rockspec

diff --git a/metalua-0.7.3-1.rockspec b/metalua-0.7.3-1.rockspec
index 77b155f..6eceffd 100644
--- a/metalua-0.7.3-1.rockspec
+++ b/metalua-0.7.3-1.rockspec
@@ -22,7 +22,9 @@
 }
 build = {
   type="builtin",
-  bin = {
-    metalua = 'bin/metalua'
+  install = {
+    bin = {
+      metalua = 'bin/metalua'
+    }
   }
 }