Link to select2-bootstrap.css in the bootstrap3 branch, not the one from master.
Replace Makefile tasks with Grunt tasks:

 * replace "make copy_docs_assets" with "grunt copy" (via grunt-contrib-copy)
 * replace "make serve_docs" with "grunt jekyll:build", "make build_docs" with "grunt jekyll:serve" (via grunt-jekyll)
 * replace "make pages" and "make pages_setup" with "grunt gh-pages" (via grunt-gh-pages); note that in contrast to "make pages", "grunt gh-pages" will not (yet) automatically push changes to origin