chore(build): Refactor rockspec to dodge luarocks 3.1.3 bug

Setting source.branch works for most cases, but when the branch is
actually a tag and not a branch then a Luarocks 3.1.3 bug rears and
tries to concatenate a nil. This dodges that bullet by setting
source.tag explicitly to not trigger the branch_or_tag concatenation.

say-scm-1.rockspec

@@ -3,17 +3,17 @@ local package_version = "scm"
 local rockspec_revision = "1"
 local github_account_name = "lunarmodules"
 local github_repo_name = package_name
-local git_checkout = package_version == "scm" and "master" or package_version
 
 rockspec_format = "3.0"
 package = package_name
 version = package_version .. "-" .. rockspec_revision
 
 source = {
-  url = "git+https://github.com/"..github_account_name.."/"..github_repo_name..".git",
-  branch = git_checkout
+  url = "git+https://github.com/"..github_account_name.."/"..github_repo_name..".git"
 }
 
+if package_version == "scm" then source.branch = "master" else source.tag = "v" .. package_version end
+
 description = {
   summary = "Lua string hashing/indexing library",
   detailed = [[