rockspec: use git+https:// for git repository URL (#29)

GitHub is going to disable unencrypted Git protocol, so `git://` URLs
will stop working soon (see [1]).

[1]: https://github.blog/2021-09-01-improving-git-protocol-security-github/

Part of tarantool/tarantool#6587

rockspecs/checks-scm-1.rockspec

@@ -1,7 +1,7 @@
 package = "checks"
 version = "scm-1"
 source = {
-    url    = 'git://github.com/tarantool/checks.git',
+    url    = 'git+https://github.com/tarantool/checks.git',
     branch = "master",
 }