Build select2-bootstrap-css on Twitter Bootstrap v3.2.0

 * update development dependencies:
   * Twitter Bootstrap from v3.0.0 to v3.2.0
   * respond.js from v1.2.0 to v1.4.2
   * jlong/sass-twitter-bootstrap to twbs/bootstrap-sass

Under the hood:

 * convert gh-pages CSS to Sass
 * replace Makefile tasks with Grunt tasks:
   * "make copy_docs_assets" with "grunt copy" (via grunt-contrib-copy)
   * "make serve_docs" with "grunt jekyll:build", "make build_docs" with "grunt jekyll:serve" (via grunt-jekyll)
   * "make pages" and "make pages_setup" with "grunt gh-pages" (via grunt-gh-pages)