Just add content into _posts/ directory and then run jekyll serve --watch to preview changes.
Install Bundler + Jekyll does not work well with permissions for /usr/bin/* on Mac OS X 10.11:
$ sudo gem install -n /usr/local/bin bundler
$ bundle install
Install just Jekyll:
$ sudo gem install -n /usr/local/bin jekyll
$ sudo gem install -n /usr/local/bin jekyll-sass-converter