Use https:// to clone templates (#1901)

Using git://-protocol for cloning git repositories does not work via
HTTP proxies. By replacing it with https:// instead it works for more
users.

middleman-cli/lib/middleman-cli/init.rb

@@ -113,7 +113,7 @@ module Middleman::Cli
     end
 
     def repository_path(repo)
-      repo.include?('://') || repo.include?('git@') ? repo : "git://github.com/#{repo}.git"
+      repo.include?('://') || repo.include?('git@') ? repo : "https://github.com/#{repo}.git"
     end
 
     # Add to CLI