Use https for ruby-install submodule

- Using git+ssh doesn't work with a deployment key since the deployment
key isn't authorized for the ruby-install repository

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "ci/docker/ruby-install"]
 	path = ci/docker/ruby-install
-	url = git@github.com:postmodern/ruby-install.git
+	url = https://github.com/postmodern/ruby-install.git