There is no need to keep rockspecs for older versions - it's enough
to ensure that `luarocks make` builds currently checked out version.
`luarocks new-version <rockspec> <version> --tag=<tag>` generates
rockspecs for releases.
Use `scm` instead of `dev` as LuaRocks 2 thinks that `dev` versions
are less than normal versions like 1.6.3, making it tricky to
use bleeding-edge version as a dependency with a constraint
such as `luafilesystem >= 1.6.3`.
