remove nvm submodule and add it from brew

Move nvm to be managed under Homebrew so it's easier to keep updated and
because having it in the dotfiles and then symlinked into the home
directory appears to cause issues with too many symlinks

.gitmodules

@@ -1,6 +1,3 @@
 [submodule "vim/vim.symlink/bundle/vundle"]
 	path = vim/vim.symlink/bundle/vundle
 	url = git@github.com:gmarik/vundle.git
-[submodule "nvm.symlink"]
-	path = nvm.symlink
-	url = https://github.com/creationix/nvm.git