filter message when deps_mode="none"

On Buildroot, we call `luarocks make --deps-mode=none --keep foo.rockspec`
So, the message about missing dependency is confusing.

```
Warning: skipping dependency checks.
...
Missing dependency for cgilua 5.1.4-1: luafilesystem >= 1.5.0
```
Merge pull request #236 from fperrad/patch-1

filter message when deps_mode="none"