Upgrade Jekyll up to v2.1.0.
Gemfile: clean dependencies.
Adjust _config.yml to the upgraded Jekyll.
Slight fixes of LESS files done.
Change the directory of Bootstrap from '_assets' to static 'assets'.
Suppress Bootstrap files from being converted by 'jekyll-assets'.
Change the directory of Tablesorter from '_assets' to static 'assets'.
Suppress Tablesorter files from being converted by 'jekyll-assets'.
Change the directory of pgn4web from '_assets' to static 'assets'.
Suppress pgn4web files from being converted by 'jekyll-assets'.