Try to change submodule as described in git book
$ git rm -r vendor/html-minifier
$ git submodule add https://github.com/kangax/html-minifier.git vendor/html-minifier
But it fails with following
rm 'vendor/html-minifier'
fatal: git rm: 'vendor/html-minifier': Is a directory
From http://txt.binnyva.com/2008/07/remove-a-folder-from-git/
git rm -r vendor/html-minifier/.
Or maybe (from https://serverfault.com/questions/256421/cant-git-rm-a-directory)
git rm --cached -r dirname