Generate the Lua code prior to determining the files in the gemspec.

Previously, qless.lua and qless-lib.lua were not getting listed in the
generated gemspec because the files didn't exist on disk at the time
the glob match occurs for s.files. This ensures that the files exist
and therefore get included in the gem.

@myronmarston, @dlecocq I'm not sure how evil you think this might be, but I'm hoping this fixes an issue that prevents qless from being used as a git dependency. It appears that shelling out from a gemspec is at least acceptable. I've seen several examples of it in other repos.

Here's two of the examples that I found in a very casual search:

https://github.com/stereobooster/submodule/blob/master/submodule.gemspec
https://github.com/saltstack/salty-vagrant/blob/develop/vagrant-salt.gemspec